The heater box is a large rectangular space extending from the firewall to the base of the windshield that houses both the heater core and fan cage opening, as well as fan cage access and coolant hose hook-ups. It protrudes into an air “plenum”, with flap-controlled slot-shaped openings in it which bypass the heater core when defrost/vent control is selected; or force air over it when heat or blower controls are engaged.
Control cables for the fresh air and defroster flaps (and heater valve) enter through a large double hole grommet located at the top right of the rear section. They must be disconnected and stored safely before disassembly can take place. Fresh air flap hinges and pivot bushings may have become rusted or worn down over time, foam may be depleted entirely, and even entire flaps could have broken into fragments; all components should be replaced prior to reassembly of the heater box.
Reverse your car into your garage, place it on blocks to raise its engine, and access the heater box mounting tabs under its cylinder heads. Use jacks to support its backside as you unbolt all the bolts securing its block and head together (it may also help drain antifreeze from radiator). Remove heater control valve by taking care to unbolt two 8mm bolts from plastic mounting bracket then loosening hose clamps leading to it. You must also disconnect its cable from its valve lever arm in order to completely disassemble its control switch as part of its disassemble process.
Pull the heater box away from the firewall while taking care not to catch it on any control cable assemblies. Fresh air flap wires (as well as defroster and vent control wires) may become caught in the heater core tube, so these should first be disconnected from their switches before being unhooked from it. Inspect the flap control rods and flaps attached inside of the box for signs of wear, corrosion or wear as soon as you open up the box and inspect for signs that they cannot close and direct air into it properly – worn flaps need either repairs or replacement before unhooking can begin.
The heater valve control cable travels through the fresh air flap opening at the top of the box, through which a small grommet allows it to pass into an opening in front of it. After reassembling, this grommet must be installed properly so as to allow its passage by this heater valve cable; alternatively it may be purchased from your local hardware store as a new replacement option.