Mustang American Autowire Wiring Harness (1965-1966) Installation Instructions

Tools Required:
  • American Autowire Single and Double Crimp Tools
  • Regular Wire Crimp Tool
  • Wire Strippers
  • Wire Snippers
  • Test Light
  • Soldering Iron
  • Zip Ties (hundreds)
  • Wire Clamps
  • Techflex F6 Braided Split Covering
  • Right Angle Drill Accessory
  • Heat Shrink Tubing
  • Electrical Tape
  • Heat Gun
  • Screw Drivers
  • Ratchet & Socket Set
  • Heat Shield Tape
Install Difficulty:
  • Difficulty
  • Difficult Project
1. To begin installing my new American Autowire 510125 Classic Update Kit Complete Wiring Harness, I took the heater box out of the car. This isn't necessary to successfully complete the installation, but I am rebuilding it and I thought now is as good a time as ever. There is a lot more space under there now.
2. While this also is not necessary, I tore the firewall insulation out of the car too. I am buying new insulation after I complete this install.
3. I reviewed the instructions that came with the kit regarding a mounting location for the fuse panel. It recommends that it is mounted offset, just to the left of the factory A/C hole on the firewall. I did remove my factory A/C because it did not work and the American Autowire kit does not support it, so this location for the fuse panel won't be an issue now, but if I decide to go with aftermarket A/C later on, a new hole for the hoses will have to be cut out. Mounting any further to the right would interfere with the heater defrost unit. I AM WAITING TO MOUNT THE FUSE PANEL AFTER I RUN ALL THE WIRES, though I pondered two different ideas for where to mount it: On the firewall or on the bottom of the cowl pan under the dash. Ultimately I chose under the cowl pan for a couple of reasons: It gave the wires a little extra travel, they did not bundle up at the top and they maneuvered much easier. It also insured that the fuse panel would not be seen, though it is still easily accessible. This was my personal preference and it can be done installed in either place. RTV sealant will need to be used on the screws and around the mounting points of the fuse panel to prevent any leaks. My car does not see water, so I am not too concerned about this.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Rear Harness
Mustang American Autowire Wiring Harness Install Image
1. I started working on this section by placing zip ties every 4" or 5" to help with routing the wires. Don't get too carried away with them because the last section of them will need to be removed once the harness reaches the trunk.
Mustang American Autowire Wiring Harness Install Image
2. I also put Techflex 3/8" cover on the rear harness. A portion will have to be removed once the harness reaches the trunk.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
3. Once the Mustang fuel sending unit wire was routed and connected, I made my way back to the trunk to begin the taillights. The instructions provided were very clear. One thing I did not use was the backup light portion of the harness or its components.
Mustang American Autowire Wiring Harness Install Image
4. The driver's side taillight requires the brown wire to be cut, stripped and connected to a double connector in the kit. The yellow wire connects to a single connector in the kit.
Mustang American Autowire Wiring Harness Install Image
5. Plug those two wires in to the supplied plug as instructed.
Mustang American Autowire Wiring Harness Install Image
6. This plug connects to the taillight socket. You will notice that the brown wire mates up with the black wire on the taillight socket and the yellow with the green wire on the taillight socket.
Mustang American Autowire Wiring Harness Install Image
7. From here, the brown wire that was split continues, with the dark green and light green wires (the light green wire is the back up wire that I did not use). I routed this group of wires along the stock location under the trunk lip. This continues through to the Mustang license plate light, where the brown wire is split again. The kit calls for a double connector, but one was not included, so I simply used male and female standard connectors to plug into the license plate light. You will notice in the picture that the brown wire is split here, connected to the license plate light and continues to the passenger taillight along with the dark green and light green (the back up light wire I did not use) wires.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
8. Follow the same procedure as you used with the driver's taillight, except you only use two single connectors. The brown wire is mated with the black wire on the Mustang taillight wiring plug and the dark green is mated with the orange wire on the taillight socket.
Mustang American Autowire Wiring Harness Install Image
9. I used a lot of Techflex and heat shrink tubing to organize wire groups and keep things neat. I have not ordered anymore heat shrink tubing, but I just placed an order for another 30' of 1/4" and 30' of 3/8" Techflex. It goes rather quickly.
10. I did not use and removed two wires from the rear harness: The orange 12v fused wire and third brake light wire.
11. The rear harness disconnect end should be left at a length where it reaches and hangs around the wiper switch behind the instrument panel for easy connection to the main harness.
Main Harness
1. After some trial and error, I found that the best way for me to do this was to lay the kit out on the ground, remove all of the wires from the harness that I knew I would not use, cover all of the branches (except the engine and light feeds) with Techflex, label it and get it nice and organized.
Mustang American Autowire Wiring Harness Install Image
2. I removed the VSS signal (401 purple), VSS ground (400 yellow) (electric Speedometer), brake light warning switch (33 tan) and stock instrument (4E brown - cut near ignition switch and shrink wrapped it) wires from the instrument cluster wires (I am running Auto Meter gauges). Circuit branch 3 contains the ground (500A black) and 12v fused (40 orange) for the fog lights. I don't have these either so I put shrink wrap on the ends of those also. I also cut the "Alternator Ign" (4D Brown) wire about 6" from the fuse panel and put shrink wrap over it. It is the exciter wire for an externally regulated alternator and is HOT when the key is on. I am running a one wire alternator.
3. There were a few other issues that I ran into: I do not have the hood turn signal connector, so they can just be left as is and hidden somewhere. The power accessory connector is one that I will never use, so it can be hidden also. They did not suggest cutting this out, just hide it. I have a MT, so the neutral safety switch wire (5 purple) will not be hooked to a neutral safety switch. According to American Autowire, the NSS wire that comes off of the ignition switch can be hooked directly to the "S" side of the solenoid. The "S" solenoid wire (6 purple) can be removed from the harness. The purple NSS switch wire has enough length to reach the solenoid. Another thing that I was confused about, but the instructions are pretty clear on, is the heater and how it is hooked up. I have a three speed heater so other applications may differ (See instructions), but the heater/ac feed wire (50 Brown) hooks to red wire on the blower motor while the other wire feeds directly to the open spade on the front of the heater box. The black wire from the blower motor to the open spade on the heater box is not supplied. A new wire will need to be run.
4. When running an Mustang electronic ignition (I have a Pertronix unit), no wires will connect to the “I” side of the starter solenoid. Application varies when running points. See instructions.
5. Once everything is organized the way you want it, you will need to insert the supplied grommets into the open harness holes on the firewall.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
6. Next, I moved on to the routing. The instructions give various routing points under the dash, but this part is personal preference, depending on the length of wire and the particular branch you are routing. I started off taking the instrument cluster wires, rear harness and headlight switch toward the front. I then ran them through the factory hooks behind the instrument bezel. I also ran the single dash light wire and plug, cigarette lighter wire and ignition switch straight from the fuse panel toward the front.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
7. The driver's side Mustang door courtesy light, quarter courtesy light and wiper feed wires all were routed up against the firewall and then around the fresh air vent. I could not get pictures of that, but you get the idea.

The driver's side courtesy lights route the same way the rear harness does. The front courtesy light housings can be mounted anywhere under the dash where there are existing holes or where the old ones were located. Here are pics of the passenger side courtesy light wire routing and the installation location of the front courtesy lights.
8. Follow the same procedure as you used with the driver's taillight, except you only use two single connectors. The brown wire is mated with the black wire on the taillight socket and the dark green is mated with the orange wire on the taillight socket.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Passenger side.
Mustang American Autowire Wiring Harness Install Image
Passenger side.
Mustang American Autowire Wiring Harness Install Image
Passenger side.
Mustang American Autowire Wiring Harness Install Image
Quarter courtesy light wire routing - Passenger side (It is the same on driver’s side).
Mustang American Autowire Wiring Harness Install Image
NOTE: The original plugs on the Mustang rear courtesy lights did not fit the connectors on the harness. I cut them and added some bullet connectors. These are plugged in when the panels are reinstalled.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Turn Signal Switch
1. Next, the Mustang turn signal switch. If you are using a stock turn signal switch and connectors, you will need to convert it to the connector provided. It is not hard, but you just need to make sure all the wires are in their correct places. The instructions do a good job of explaining, but there were a couple of wire color differences from my stock switch and the colors stated in the instructions. I won't name all of the colors because they are in the instructions, but here are the differences in the colors I experienced:

- American Autowire instructions said the stock wire color from Ford for the Left rear turn was Green with orange stripe. My stock wire was Green with red stripe.
- It also said the Right rear turn wire was Orange with a blue stripe. Mine was just orange.
- Please note the two color discrepancies were on the two wire connector from the stock switch.

On page three of the instructions it says how to convert them. Again, triple check which slots you put those wires in the new connector. They must coincide with the end you are connecting it to. This is a short run of wires so there are limited areas to route the switch wiring. It has a ground wire that should be grounded nearby.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Brake Light, Ignition, Headlight and Dimmer Switches
1. The brake light wires go on to the Mustang brake light switch above the brake pedal. This simply presses on the switch. It comes with the plug connected.
Mustang American Autowire Wiring Harness Install Image
2. The kit comes with a new ignition switch, headlight switch, and headlight dimmer switch. Unfortunately, I just replaced these not too long ago, but American Autowire says that their switches have new technology in them, so I replaced them again. While I won't go into detail regarding the switch replacement instructions, one thing I do recommend is putting the headlight switch connector on the switch itself before installing the switch in the dash. Same goes for the headlight dimmer on the floor. There is very little room with the size of the plug that comes on the wires. Since these connectors are all new, they are sometimes a bit difficult to put on the switches. Not pictured, but the cigarette lighter wire just simply slides onto the back of the lighter.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Engine Bay and Lights
1. At this point, I have not mounted the fuse panel. The routing of the engine bay and headlight wires can run up against the firewall and through the grommets we installed earlier. The driver’s grommet is for lights and middle grommet is for the engine harness. Inside the cab I tried to put Techflex around each wire feed up to the firewall before I fed the wires through the firewall. It is difficult to reach back up there after you push the wires through.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
2. At this point, once everything inside was routed properly and the engine and light feeds were through the firewall, I mounted the fuse panel, relay panel and accessory feed. The relay block has a ground that I located on the firewall. I hooked the accessory feed up, out of the way using the supplied “J” hook. If you elect to use this mounting location, be sure to put the RTV sealant on the screws before screwing them in and put dabs around the base of the mounting locations.
Mustang American Autowire Wiring Harness Install Image
3. The new kit comes with separate ground wires for the headlights. Those need to be attached to a good chassis ground. The horn wire was not as long as it should be and the instructions only show there being one location for the horn, BUT there are two Mustang horns. So, I piggybacked the wires using a double connector and no plastic plug (the instructions called for a single plastic plug and connector, but the double connector would not fit in the plastic clip) from the driver's horn and began routing to the passenger horn. There is not a separate ground for the horns; it is grounded through the relay. I left the electric fan wire in place, terminated the end and stuffed it in the covering. I snipped the fog light wire near the radiator support and terminated the end of it as well. I put it in the covering, too.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
4. I bought new Mustang parking lights and the new wires were a different color, so here are the proper connections. The pics apply to both sides, except for the piggy back connections from the brown parking light wire on the driver’s parking light to the passenger parking light
Wire from American Autowire.
Mustang American Autowire Wiring Harness Install Image
Wire from American Autowire kit. Left/Driver Parking light. Piggy backed to lead to Right/Passenger Parking Light
Mustang American Autowire Wiring Harness Install Image
Picture of stock parking lights before my connections were made.
Mustang American Autowire Wiring Harness Install Image
This is after I made my connections on stock parking light wires. It is the same for both sides.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
The wires from the parking lights will be inserted into the supplied connector and mated with the female connector from the kit wires, not the ground location. This is the same for both sides.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Connection completed. This is the same for both sides, except the driver’s side parking light wire is piggy backed to the passenger side.
Mustang American Autowire Wiring Harness Install Image
5. Once the driver’s lights are complete, I moved my way to the two horns, passenger headlight, and parking light.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Driver and Passenger side.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Passenger horn branch.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Passenger headlight connection is complete. The bundle of wires that the arrow is pointing to are the parking and turn signal wires. They run right behind the headlight bowl down to the parking lights. It is the same on both sides. The lights are complete at this point.
Mustang American Autowire Wiring Harness Install Image
Engine Harness
1. The American Autowire kit is setup for a one wire alternator and that is how my connections are made. It can be used for points, but is not encouraged. There are separate instructions in the kit for this type of connection.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
2. Here are the routing pics. I will show the connections next.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
The Mustang 3-speed heater blower motor requires an additional wire to hook to the black wire on the blower motor and then to the single spade on the resistor on the front of the heater box. The additional wire is not supplied and will need to be routed here.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
3. Connections in the engine bay.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
This is not necessary, but I thought I would give it a try.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
This Mustang ground cable does not come with the kit, but I recommend getting a new one.
Mustang American Autowire Wiring Harness Install Image
4. I placed the fuses in the panel just as they were pictured in the instructions. Some of these are not being used.
Mustang American Autowire Wiring Harness Install Image
Gauge Cluster
1. I am running Auto Meter gauges in a stock 5 gauge bezel using CJ's Mustang gauge support. The American Autowire kit supplies a quick disconnect and a lot of wire. One thing these 5 gauge bezels do not have is a high beam indicator light. I had an idea to go to the local auto parts store and pick up a small indicator light. I simply located a good spot for it, drilled a hole using the bit size indicated in the instructions, and it popped right in. It works great, too. The key to this part of the job is to make sure the 12V wire, gauge ground wire, light power wire, light ground wire all get piggy backed correctly from one gauge to the next and ultimately end as one wire at the end to plug into the quick disconnect. I soldered these connections to insure they would last. It is difficult to explain the procedure but the pictures tell the story. See below.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
- I started the pink 12v wire at the fuel gauge and piggy backed that wire to each gauge which leads to the harness disconnect. This wire connects to the "I" post of each Auto Meter gauge.
- Same for Ground. "G" on gauges.
- The fuel gauge sending unit wire connects to fuel gauge on the "S" post. Route this to the end.
- The oil pressure sending unit wire is next. Connect to the "S" post. Route this to the end..
- Next is the volt gauge. It only requires a 12v and ground wire.
- Finally, do the water temperature gauge. This wire goes on the "S" side as well. Route the wire to the end.
- The Left & Right blinker lights have their own dedicated wires. Connect these to their respective sides. Since the gauge support for my Auto Meter gauges is plastic, I bought two Auto Meter spare bulb sockets and modified the blinker holes on the support plate to hold them (super glue too). The white wires connect to their respective blinker wires and then a ground wire.
- The high beam indicator light has a dedicated High Beam Indicator light wire. One wire is a ground and the other connects to the high beam indicator light wire. The indicator wire leads to the quick disconnect.
- The bulb wires and their grounds need to be connected in line with one another and leads to the end of the harness. I soldered these in line starting with the ground wires. This group leads to the end of the harness and connects to the gauge ground piggy backs, then to the quick disconnect. Do the same for the light wires and connect these with the gray dash light wire to go to the quick disconnect. This part requires some creativity and thought. It can be done multiples ways, but I just started connecting each ground wire in a series the best way I could. Then do the light wires. I soldered the connections and put heat shrink wrap on the connections.
- Once everything is connected, cut the wires to the length. You will need and use the provided spade connectors. Slide each one in the male side plug.
NOTE: I cut the gauge wires under the dash to a length that would give me a little extra room. I also cut in a place where I could just reach under the dash and disconnect the cluster instead of only being able to access it from directly behind the instrument panel.
Under dash pics of the gauge wires.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
I installed the Techflex on the back of the cluster for a little protection. I did this after I tested everything. The covering is zip tied on there very lightly so there was not too much extra pressure on the connections.
Tach, Clock, Radio
1. This part may differ for most people, but the general idea is the same. If you do not have a tach and clock like I do then the Mustang radio connection is very straight forward. My tach requires a 12v switched source and the clock requires a 12v constant source. The radio supplies both of these. My tach and clock are dash mounted. I tied these in with #43, tan, 12v switched, RADIO wire AND the #99, yellow, 12v constant, RADIO BAT wire. Simply piggyback these connections, then use one of the extra supplied quick disconnects for a clean connection. I had to use a little extra wire for the lights of the tach and clock. I also had to supply my own ground, since I separated the tach wire from the main instrument cluster disconnect. The clock, tach, and radio need a ground wire anyway. It all works properly and was not hard to do. I did not use supplied connectors. I used my own connectors and put heat shrink wrap around them, then Techflex. Another thing that is nice, the American Autowire kit supplies an extra light feed wire, gray, 8b. That's where I lit my clock and tach from. All of these wires need to be mated with the supplied spades and the other end of the connector (Female end).
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Wiper Connection
1. The wiper setup was pretty straight forward. I did purchase a new windshield wiper wiring harness from CJ's. It routes right behind the instrument cluster and in via the factory plastic hooks. The harness hooks to the factory locations on the motor and ground. The supplied wiper feed wire/connector attaches to the Orange/White power wire on the wiper harness. This wiper harness had a washer motor connection on it. I am not using it, so I zip tied it and tucked it away. The additional white wiper feed connection on the supplied connector is for that Mustang windshield wiper motor. I put a little extra heat shrink wrap around it and zip tied it up.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
Heater Connection
1. The Heater/AC feed wire can now be connected to the red wire on the heater blower motor. The additional wire that we ran from the black wire on the heater motor in the engine bay will now need to be connected to the single spade on the front of the heater box. I used a standard female connector for this.
Mustang American Autowire Wiring Harness Install Image
Mustang American Autowire Wiring Harness Install Image
To the front of the Mustang heater box.
Mustang American Autowire Wiring Harness Install Image
Note the connections here. The heater control switch needs to be plugged back in. I purchased a new Mustang heater control assembly from CJ's.
Mustang American Autowire Wiring Harness Install Image
Conclusion
Test all components. If there are things that are not working, then check all the grounds and connections again. Everything worked on my first attempt, so hopefully you will have the same luck! The installation was not terribly difficult, just time consuming with one person. Hopefully, these instructions have eliminated a lot of trial and error in your installation of your American Autowire Classic Update Kit Complete Wiring Harness. Each installation will vary a bit from person to person and car to car. All in all this was a great experience and I would definitely do it again on my next car. For your information, the American Autowire single crimp tool and American Autowire double crimp tool are a must for this project!