I have a problem. 79 bronco, all stock. Its cold here in MI and the heater works good and would work better if it was not for the cold air coming in around the passenger window. (needs new track rubbers seals) Here is the problem. The windshield is clear, no frost from the night before, sun up or sun down, morning or night does not matter. I can drive about 7 or 8 block and the INSIDE of the windshield will frost over so heavy that I can not see out of it. I can use the ice scrapper on the inside. It will frost if I have the heater on. If I leave the heater off it does not frost. WHY?