Son of a Hot

Are the doors vacuum or cable operated? I bet if you pull the box apart you will find 30 years of Georgia pine needles and a rat nest blocking the air flow of the heater core. While its out clean and reseal your doors with closed cell foam. If it is vacuum op, then use a vacuum pump to check the pots function. If it is a cable op, soak the cables in diesel fuel to free them up. by servicing the factory unit and replacing/upgrading the missing parts you'll come out cheaper then replacing the whole unit.