If fog and sweet smells are filling your Mustang’s cabin and the source is not donuts and coffee, your heater box could be in trouble. Use this step-by-step guide and video to rebuild or replace your classic Mustang’s heater box. We’ll show you how it’s done using a 1965-1966 Mustang heater box rebuild kit and a new heater box assembly.
Before You Get Started
Replacing the heater box or core is the perfect time to update some of the other components in your heating system. Consider replacing the heater motor, hoses, and hardware to avoid having to remove and reinstall the box a second time.
If you like to have as much room as possible while you work, remove the passenger-side seat. This will leave plenty of room for you to maneuver freely while you work under the dash. You can also pull the glove box out of its housing to reach hard-to-see places.
Don’t forget to disconnect the battery before removing the heater motor’s wires. Also, drain the coolant from the radiator. The last thing you want is for coolant to stain or damage your Mustang’s carpeting when disconnecting the hoses.
Rebuilding Your Heater Box
Removing and rebuilding your ‘65 or ‘66 Mustang’s heater box isn’t too difficult. The key is to stay organized, and be ready to replace the entire heater box if the original is in rougher shape than you expect.
- Basic Screwdriver Set
- Basic Wrench and Socket Set
- Hammer (Optional)
Step 1: Disconnect Heater Hoses
Under the hood, disconnect the heater hoses from the heater core.
Step 2: Unbolt Heater Motor and Disconnect Wire Harness
Remove the bolts holding the heater motor to the firewall. Then, disconnect the heater motor wires.
Step 3: Inspect Defroster Ducts and Hoses
Remove the two screws holding your dash speaker grille in place and pull the grille out. This will allow you to examine the defroster ducts and decide whether they need to be replaced.
These ducts attach to the defroster hoses which run to the heater box plenum. If the ducts look good, be sure to check the hoses under the dash as well in case they’re tattered and need replacing.
If you plan on replacing the ducts, or if yours are missing, set the new ducts into the holes next to the speakers. This will keep them safe and out of the way while you install the new box.
Step 4: Remove the Heater Box Retention Screw
Remove the single ⅜” screw holding the heater box in place. This screw is under the dash all the way at the top of the heater box.
Step 5: Free the Box
With the retention screw removed, shimmy the box out and let it rest on the floor under the dash. Then pull the defroster hoses free from the plenum.
Step 6: Remove Heater Box Cables
Pull the small end caps off of the passenger-side cable arms with pliers. Remove the set screws on both of the passenger-side cables and pull the cables off the arms. Repeat this step for the driver-side cable.
Step 7: Pull Heater Box Out
With the cables unhooked, the heater box screw removed, and the defroster hoses disconnected, there should be nothing holding the heater box in. Carefully pull the box out from underneath the dash.
Step 8: Remove Heater Box Controls
Remove the two nuts holding the heater box controls in place. Pull the controls and cables out through the housing. Bring the heater control assembly and cables to your workspace.
Step 9: Decide Whether to Rebuild or Replace the Heater Box
Examine your heater box and make a decision on whether you want to rebuild or replace it. If the box is cracked, rusted, or falling apart, you’ll want to replace it. If everything is still in working order and just needs new seals and a new core, rebuilding is a great option.
If you’re going to rebuild the box, continue following each step in order.
If you’re going to replace the box, skip to Step 14, follow Step 15, then go to Step 26.
Step 10: Remove Hoses
Loosen the retention collar on each heater box hose and remove them both. Now is the time to check the hoses and decide whether you want to replace them or not.
Step 11: Remove Main Heater Box Seal
Remove the large circular seal from around the mouth of the box. You may need to tear it off piece by piece if it’s dry-rotted in place.
Step 12: Remove Clips Holding Box Together
Using a flathead screwdriver, remove the clips holding the two halves of the heater box together and pull the box apart.
Step 13: Remove Mustang Heater Core
Pull the old heater core out of its housing.
Pro Tip: Instead of throwing your heater core away, consider recycling it. This is a great way to offset the cost of your upgrade.
Step 14: Remove Factory Heater Core Seals
Remove the seals from both halves of the heater core housing. These are often dry-rotted in place and might require the use of a flathead screwdriver for removal.
After the old seals are gone, give the heater core housing a good cleaning.
Step 15: Replace Heater Core Seals
The small half-moon cutouts on the seals should be at the holes where the heater core hoses will go. Spray some adhesive onto both of the heater core seal locations and press the seals in place.
If you’re replacing your heater box, you can skip to Step 26.
Step 16: Remove Heater Motor
Remove the four nuts holding the heater motor to the box and pull the assembly out.
Step 17: Replace Motor Seal
Remove the remnants of the old seal from around the motor assembly. Then remove any rust or corrosion and set the new seal in place.
Step 18: Replace Squirrel Cage Seal
Replace the flat ring seal that sits between the squirrel cage and motor. Be sure to clean the cage before you put on the new seal.
Step 19: Replace Heater Box Door Gasket
Clean the heater box door, taking care to remove any remaining pieces of the original gasket. Spray a light coating of adhesive on the factory heater box door and press the gasket in place.
Note: If your kit is compatible with multiple years, the gasket with the cutout at the corner is the one for ‘65-’66 Mustangs.
Step 20: Replace First Blend Door Gaskets
Clean both sides of the blend door and remove the OEM gaskets. Spray a layer of adhesive on each side of the door and press the larger rectangular gaskets in place.
Step 21: Clean Second Blend Door Assembly
Remove the two screws holding the second blend door assembly to the heater box and pull the assembly out. Clean each surface and make sure to remove any leftover factory gaskets.
The assembly will need to be cleaned all over, but be sure to focus on the top, sides, and door.
Step 22: Replace Top Gasket
Spray adhesive onto the surface at the top of the assembly that has the holes for the retention screws. Set the thin bar-shaped gasket with the two holes in place and press it into the adhesive.
Step 23: Replace Second Blend Door Gaskets
Spray adhesive onto the front of the blend door and press the small rectangular gasket onto the glue. Repeat this step for the back of the blend door.
Step 24: Replace Side Gaskets
Spray adhesive onto the sides of the assembly and press the triangular gaskets in place.
Note: These gaskets may need to be trimmed for a perfect fit.
Step 25: Reinstall Second Blend Door Assembly
Once the seals and gaskets are in place, reinstall the second blend door assembly into the heater box.
Step 26: Install New Heater Core
Position the new heater core into the housing.
Step 27: Reassemble Heater Box
Reassemble the heater box and reinstall the retention clips.
Pro Tip: If you’re having a hard time getting the clips back on the box, a light tap from a hammer should do the trick. If you’re nervous about damaging the box, lightly tap the clip on the ground with a hammer to widen it. This should make it easier to clip the box together by hand.
Step 28: Reinstall Heater Motor
Set the squirrel cage back into its housing on the heater box. Feed the four bolts on the box into the four holes on the motor’s lip and secure each bolt with a nut.
Step 29: Reinstall Plenum
Feed the plenum onto the outlet near the motor assembly and secure it in place with the factory hardware.
Step 30: Replace Main Seal
Replace the large circular seal around the opening on the heater box on the side across from the plenum.
Step 31: Remove Factory Heater Control Cables
Remove the screw holding the longest cable’s clamp in place on the heater box control assembly. Then remove the clamp and the old cable.
Step 32: Install New Factory Heater Control Cables
Feed the new longest cable’s wire end into the hole that the original cable was removed from. Position the wire into the groove and feed the clamp back into the slot.
Tighten down the clamp with the original screw. Take care to not push the exposed spring too far under the clamp (there is a stopper present, just be sure to use it).
Repeat these steps for the other two cables. The mid-length cable goes in the middle and the shortest cable is last.
Step 33: Reinstall Heater Box Controls
Feed the cables through the heater box control housing and reinstall the controls.
Step 34: Position Heater Box
Attach the original or new hoses to the heater core. Feed the hoses through the firewall with the heater box wiring and slide the box back in place under the dash.
Step 35: Reconnect Cables to Box
Reconnect the cables to the heater box. The longest cable goes to the passenger-side front mount arm. The mid-length cable goes to the arm behind the passenger-side front cable. And the shortest cable goes to the driver-side arm.
Secure each cable in place with the mounting tabs and reinstall the caps on the tops of the arms.
Step 36: Install Defroster Ducts
Feed the defroster ducts into the defroster hoses. Set the hose and duct assemblies into the holes on either side of the dash speakers.
Step 37: Secure Defroster Ducts
Secure the ducts in place with the retention clips. Depending on your kit, these clips might be included. But if not, use the OEM hardware.
Step 38: Connect Defroster Hoses
Extend the hoses down through the dash and connect them to the plenum. The right hose goes to the right hole on the plenum and the left hose goes to the left hole.
Step 39: Reinstall Dash Speaker Grille
Set the grille back into its housing and secure it in place with the retention screws.
Step 40: Finish the Job
Tighten the ⅜” retention screw securing the box under the dash. Then, tighten the bolts that connect the heater motor to the firewall in the engine bay. Make your final electrical connections and reconnect the battery.
Also, don’t forget to reconnect your heater hoses and refill your Mustang’s coolant reservoir. Once everything is secure and connected, reinstall the passenger-side seat and the glove box.
Bring On the Heat
With a fully-functioning heater box, you should be getting a consistent flow of warm air when the switch is on. If the fog has cleared and the sweet smell of coolant is gone, you should be good to go.
This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.