I have a 2003 Pontiac Montana van. When I now turn my heat on to defrost my windows, my van fills with moisture. The front window will stay clear but now all the rest fog up and they will then freeze so need to be scraped from the inside just to see out. One mechanic shop charged me $508 to replace my thermostat which made no difference. It only happens when driving the van, not when it is sitting. Can u help?