This so totally sounds like the flaps inside the heater assembly arent switching. If you dive under your dash and follow from above the radio back along the vent, you'll see these "drums" with vacuum lines going to them. There's a little flap that gets opened and closed by a lever right where you're looking and this lever is actuated by the vacuum drum. This baby redirects air to either the vents or the windshield. There's a second flap sort of behind the glovebox that switches from the heater core passage to the a/c and heater passage. I think this one is cable actuated. There's also a third flap by the passenger side kick panel that directs air down.
When you turn the selector knob, do you hear a hiss....creak....flap? You should. If it's not switching, you may have a leak in one of those lines. They end in the back of the control knob unit. I'm not positive where the vacuum comes from, actually, but it does work with the engine off.
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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