Mustang Heater Rebuild Kit (1965-1966) Installation Instructions

Tools Required:
  • Ratchet & Socket Set
  • Flat & Phillips Head Screw Drivers
  • Towels
  • 3M Adhesive
  • Degreaser
  • Drain Pan for Antifreeze
  • Box Cutter
  • Vise Grips
  • Spare Screws
Install Difficulty:
  • Moderate
  • Moderate Project
1. To begin the installation of your Mustang Heater Rebuild Kit, disconnect the battery.
2. Drain the antifreeze out of your Mustang radiator. There will be a mess near the heater hoses on the water pump and intake, so prepare with towels.
3. Unplug the two wires (some may have three) from the blower motor located on the passenger side of the engine bay.
Mustang Heater Rebuild Kit Install Image
4. Loosen the clamps on the Mustang heater hoses (intake and water pump hoses). Place towels under the hoses to prevent a mess. Remove hoses. (NOTE: The water pump hose leads to the top nipple of heater core and the intake hose leads to the bottom nipple of heater core).
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
5. Remove the four nuts that hold the blower motor in place and set aside. Two are shown and two more are on the bottom side.
Mustang Heater Rebuild Kit Install Image
6. Moving inside the car, I found it easier to see everything with the glove box out. There are seven screws that hold it in place and one that holds the cable. Remove them all and set aside. The glove box takes some maneuvering, but should slide out from the bottom.
7. There is a screw that mounts the heater box on the cowl under the dash. Remove it and set aside. Unplug the Mustang heater blower motor switch from the front of the heater box.
Mustang Heater Rebuild Kit Install Image
8. Loosen the Mustang heater hose clamps that are located on the back left side of the heater box. Again, place towels underneath to prevent a mess. Those will slide off. I elected to replace my heater hoses while doing this project.
Mustang Heater Rebuild Kit Install Image
9. This kit also comes with new Mustang heater cables, so the old ones must be removed. I elected to pull the instrument bezel out for easier access. In addition, the nuts that hold the heater cables in place on the heater box were a bit difficult to get to from under the dash. Each cable links to a lever on the heater control. These just slide out when moved to the proper angle (The cables have a zig zag that holds them on the control levers).
Mustang Heater Rebuild Kit Install Image
10. At this point, the Mustang heater blower motor is loose from the firewall, the hoses are off, the one screw that mounts the heater box to the cowl under the dash is off and the heater cables are loose. The heater box should slide out. The fiberglass is brittle from its age, so be careful not to bang it around. The heater core will still have antifreeze in it, so be careful.
Mustang Heater Rebuild Kit Install Image
11. Pull the box out and set it on the ground. There are nine or ten clips that hold the box together. Take a flat head screw driver and pull them off. I had a few pieces of fiberglass break off so be as careful as you can.
12. Once the clips are off, the box will come apart. The Mustang heater core can be removed at this point. The blower motor is still held in by four nuts. Remove these. The motor will slide out. To remove the nut holding on the heater cables, I elected to take vise grips and grab on to it, not using a lot of pressure, and turn counter clockwise. They eventually came off. Be careful not to damage these. At this point in time, it would be a good opportunity to clean up your heater box and remove any debris. I had about 40 years of leaves and grime in mine. You will need to do this anyway for the new seals.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
13. Now for the Mustang heater seal kit that come in the kit! The package comes with a decent picture of the placement. Again, make sure everything is thoroughly cleaned. I bought some of this 3M adhesive and it worked great. Spray each surface, seal, and location on the heater box, and let sit for a minute before mating the two together. It will stick better this way.
Mustang Heater Rebuild Kit Install Image
One seal on inside front door of heater box.
Mustang Heater Rebuild Kit Install Image
Trap door right behind the front door of heater box. One seal on each side.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Plate that is located on resistor side of heater box where the heater core rests. One seal there.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Unscrew the two screws located on the blower motor side of heater box and the piece with trap doors will slide out.
Mustang Heater Rebuild Kit Install Image
There is a seal on either side of trap door.
Mustang Heater Rebuild Kit Install Image
There are two small strip seals on the side facing middle of box.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
One wedge shaped seal goes on top of this piece and bottom. I trimmed a groove out to make room for the trap door mechanism.
Mustang Heater Rebuild Kit Install Image
One strip goes on the side that the screws go in.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Place this entire piece back in just as you removed it. It is a tight squeeze now that you have these seals on the top and bottom. Gently spread the box apart with your hands and maneuver this piece back in.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Screw the two screws back in to hold in place.
Mustang Heater Rebuild Kit Install Image
Now place the other seal on the other side of where the heater core rests.
Mustang Heater Rebuild Kit Install Image
Slide the heater core back in the heater box as it came out (not pictured, but there is only one way it can go in) and then mate the two heater box pieces together and place the clamps back on. I used vise grips, with very light pressure to prevent fiberglass cracking, to assist in holding the two sides together.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Once the box is back together, you can install the last seal around the circular opening on the heater box.
Mustang Heater Rebuild Kit Install Image
14. Once the Mustang heater box is back together, you can reinstall the blower motor. There is one seal that fits on the motor. This joins the firewall.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
15. The defrost box can now be placed in the proper location on the heater box. There are four screws: two on top and two on bottom.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
16. The heater cables can now be reattached to the heater box and the defroster. Tighten the Mustang heater cable clamp back down and reinstall those nuts on the top of the trap door mechanism. I just set it on there and tapped with a hammer. Once the cables are back in place you are ready to move the box back to the car and reinstall it just as it came out. DON’T FORGET TO CLAMP THE HOSES BACK IN PLACE ON THE HEATER CORE AND REFILL THE RADIATOR WITH ANTIFREEZE! I have not reinstalled my unit yet because I am in the middle of rewiring the entire car. For me, it was easier to leave it out.
Mustang Heater Rebuild Kit Install Image
17. Once the unit is back in the car and properly mounted, just as it came out, you can reconnect the cables to the Mustang heater control assembly. Best access is through the instrument bezel opening. They simply hook into each lever with the zig zag shape of the cable and then the clips are screwed back in to secure them. The defrost cable is the shortest and belongs on the lever labeled DEF and then hooked to the front of the plastic defrost box. The TEMP cable is the longest and belongs on the front trap door mechanism of the heater box. The HEAT cable is the middle length one and belongs on the rear mechanism of the heater box.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
18. With the heater in place, you are ready to connect the Mustang defroster hoses from the defrost unit to the underside of the dash speaker grill with the supplied clips and mounting plates. These clips will hang on the defrost openings under the speaker grill. The speaker grill will need to be removed by loosening the two screws that hold it in place.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
19. You are now ready to plug the heater control switch back to the Mustang heater blower motor switch resistor on the front of the heater box.
Mustang Heater Rebuild Kit Install Image
Mustang Heater Rebuild Kit Install Image
20. Remember to check all steps that we did to remove the unit after you put it back in to make sure everything is absolutely correct before you start your car. Once you're sure, you are ready to fire it up to test your new Mustang Heater Rebuild Kit.