Rusty, frozen heater boxes.
The reason it's only on one side is because your whole heating system is split into two pieces (right and left) with one chain of components (heat exchangers, ducts, heater boxes) servicing each half of the car. So if the passenger-side box has gotten stuck in the open position, you'll have a constant flow of engine-heated air on one side. The hot air is propelled by a blower in the engine compartment, not the fan that controls the velocity of fresh air from the vent under the windshield.
A quick patch for your problem is to lift up the carpet in the passenger's foot well, and unscrew the hose clamp that connects the flexible ducting to the sheet metal tube right on the right side of the foot well. Slide a piece of 4-inch square cardboard in there, and let the pressure from the flexible ductinghold it in place. Put the carpet back, and no more hot air flow. Of course, the downside to this is that there's no heat or defrost at all on that side, now. But it beats cooking non-stop if you're in a warm part of the country.
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Jack Olsen
1973 911 T sunroof coupe
[This message has been edited by JackOlsen (edited 10-14-2000).]