Stopping the Heater!
 

Heat flows into the interior of my 1990. The little valve at the back of the pass. side engine compartment moves as it should. HOW can I stop the heat? Also, there isn't much outside airflow into the interior.

Hope this is something simple.

make sure the temp setting is set to 65

First thing I checked.

I've noticed if you have the climate control set to let in outside air and not recirculate, it allows air to come in while the car is moving. If you have it set to warm, this could let air warmed by the heater core into the cabin.

The digital temp. is set all the way down to 65. There is probably something wrong with the air inlet valves/baffles/whatever they use. I hope it's just a vacuum line or something simple.

Start your car, have your buddy adjust the temp gauge from 65 to 80 or 85 or whatever the top is (hanv't driven my baby for awhile) and you look at the engine compartment, specifically in the rear most corner on the passenger side. There is a valve there, a VSV (Vacuum Solenoid Valve) type. When you adjust the temp inside you should see the little arm on the bottom of the VSV move up or down, if its not moving then that is (one of) your problem(s). Its probably stuck in the open position and constantly letting heat into the heater core. Is the air coming out of your vents HOT or WARM? Hot indicates heat getting to the heater core and warm might be blow by from the cowl induction. Heat from the engine bay gets sucked into the vents or even pushed into the vents when your driving, just an unfortunate side effect of owning a supra. Let us know about the VSV, usually when they fail they get stuck shut, but who knows.

Yours might even be bypassing, which is a simple enough check, just pull it off and try to blow air through it, if you can then its a leaky valve. Good luck.