Mustang 3-Speed Heater Box Assembly (1965-1966) Installation Instruction

Tools Required:
  • Ratchet & Socket Set
  • Screw Drivers
  • Towels
  • Box Cutter
  • Vise Grips
  • Spare Screws
Install Difficulty:
  • Moderate
  • Moderate Project
1. The first step in the installation of the new Mustang 3-Speed Heater Box Assembly was to add the heater core to the new heater box. To do so, the 11 clips that hold the box together had to be removed. After the clips are removed, the foam around the round opening is removed and the two halves of the heater box are separated.
Mustang Heater Box Assembly Install Image
2. The Mustang heater core was put into the heater box along with the parts provided with the heater box (padding both below and above the heater core along with the bracket). A screw driver is holding up the core so you could see the padding both below and above the heater core.
Mustang Heater Box Assembly Install Image
3. Add the Mustang heater housing clips back to put the two sides of the heater box back together. This does take some effort. To hold the two halves together, I used vice grips along with an old rag so I didn’t scratch the new box. If you are doing this alone, this is a must. If you have an assistant, you might be able to have the assistant hold it together while you push the clips back on (or vice-versa).
Mustang Heater Box Assembly Install Image
4. The rubber foam can also be reattached. Here is a picture shown without the foam attached. See below to see the foam on it.
Mustang Heater Box Assembly Install Image
5. Install the Mustang heater plenum by pushing the plenum over the heater box and then installing the four screws that will hold it in place.
Mustang Heater Box Assembly Install Image
6. Attach Mustang hoses (measure beforehand to make sure you have enough hose), then attach the defroster hoses and attach the heater motor with the wheel going inside the heater box.
Mustang Heater Box Assembly Install Image
7. Attach the defroster/heater cables. The smaller of the cables is attached to the end of the heater box as shown below.
Mustang Heater Box Assembly Install Image
The longest two defroster/heater cables are then attached as shown below.
Mustang Heater Box Assembly Install Image
8. Take the heater box and first push the heater hoses through the holes in the firewall and then push the heater box into place by aligning the four bots shown above through the four bolts holes in the firewall. The four nuts that go on the bolts of the heater box will hold this into place. This is really a two person job. Have one person holding the heater box under the dash while another person attaches the nuts to the bolts that are protruding from the firewall.
Mustang Heater Box Assembly Install Image
9. You also have a bolt to attach under you dash inside the car to hold the heater box in place.
Mustang Heater Box Assembly Install Image
10. Attach the bottom heater hose (bottom at the firewall) to the Mustang intake manifold.
Mustang Heater Box Assembly Install Image
11. Attach the top heater hose (top at the firewall), to the Mustang water pump.
Mustang Heater Box Assembly Install Image
12. Finally, plug in the heater cables back onto the heater switch and the wires and you are ready to roll with your new Mustang 3-Speed Heater Box Assembly.