Heater core replacement is quite simple on 1965-1968 Mustangs. The early Mustang uses a very simple "radiator" style heater core that can effectively warm you and keep your windows defrosted on cold winter days. The heater core is located in a blower housing box situated under the dash on the passenger side of the car. A fan blows air through the heater core and sends the hot air through a plenum that directs air to your defrosters or to the floor.

The first step is to remove the two heater hoses from the engine. Place a can under the hoses to catch any fluids. Then unplug the two wires connecting the heater blower motor to wire harness. You will notice 3 nuts on the engine fire wall next to the wire leads from the blower motor. These 3 nuts retain the heater core assembly to the firewall. Remove all 3 nuts.

Go into the car and look under dash. You will see heater control cables attached to the plenum and heater box. Remove and save fasteners. Unplug the wire harness from heater box. You will notice a simple bracket and screw attaching blower box to underside of dash, remove screw. Lift off defroster ducts from heater plenum.

Gently remove complete heater assembly from under dash. Once you have the assembly out you will notice a number of clips (C0DF-19A779A) holding the blower assembly together. Carefully remove clips and separate housing. You can now remove old heater core and hoses , clean the inside of the box from debris and remove the old gaskets and foam seals.

Using a new heater foam kit, simply replace all gaskets and foam as required. We recommend using 3M Super Weatherstrip Adhesive (08008) for internal seals and Permatex Form-A-Gasket (2AR) between heater blower motor and large round gasket that makes contact with inside of firewall. If your heater blower motor makes noise (or no noise at all ! ) replace the motor now. Re-assemble the heater box and install a new plenum.

Talking your heater hose in hand, fold the hose in half and place each end through the two heater hose holes in the firewall from under the hood. Gently slide the hose over the heater core ends protruding from the heater box and install new clamps. Slide the heater box assembly into position with the blower motor and attaching bolts through firewall. Be sure the heater hose does not crimp and it goes through firewall smoothly. Re-install heater control cables, and plug heater thermostat wire harness to box. Fasten heater box bracket to dash.

Back in the engine compartment, plug blower motor wires to main harness. Stretch out the looped heater hose and cut to fit (not tight!) to engine. Check your antifreeze level in radiator and you are done!!!!!

Below are some common trouble shooting Heater Core tips:

You can hear the fan blow, but no air is coming out of the vents, or only a little. Look at the factory cardboard heater plenum. After 30+ years these will deteriorate and scatter the air flow. The replacement plenums are plastic and will last indefinitely. Also check the dash defroster ducts. These also came factory equipped with cardboard or paper vents that will fall to the floor with age. The replacement ducts are plastic.

If you can hear the fan blow, air is coming out of the vents, but it is cold. There are two possible problems.

Check the thermostat (watching temperature gauge) to see if the temperature raises and then drops. If you have a defective thermostat the engine will not circulate water through the heater core. The engine temperature will drop to below normal operating levels once the thermostat opens. We see alot of stuck thermostats in extreme cold weather.
The heater core is probably clogged with hard water deposits or leaves. You must replace the heater core.

If water is dripping on the floor, most likely your heater core has damage to the inlet/outlet pipes due to movement of the heater hoses in the engine compartment. You must replace the core.

If you can hear the blower motor. Your blower motor is exhausted. You will need to replace the motor when you have the blower case apart for heater core replacement.

Parts required for heater box restoration:
(See online store for current prices)

• 65-73 Heater Core 34.95
• 65-73 Heater core foam kits 14.95 • 65-73 Heater hose with clamps 19.95
• 65-68 w/o AC Heater plenum. 24.95 • 65-68 w/o AC Defroster ducts. 24.95
• Heater control cable sets, from 24.95
• Blower motors from 59.95

Photo: Plenum, Hose and Foam Kit