I have a 1992 Pontiac firebird 3.1 hot air comes in when I am driving down the road and I was wondering if there is a way I can close the circulation door so that no air comes in while driving

If your car has AC, the vents are closed when you select "OFF" with the slide switch on the dash. If is does not shut off the flow, then the manifold on the back of the unit is probably going bad. It shifts the vacuum from the engine to move the vent baffles. Go to "www.thirdgen.org" and do a search for your problem. There is a simple fix by replacing a screw on the manifold but you have to remove the heater control panel to get to it. The original part breaks with age - its plastic. I did it on my '91 a month or so back.