This is a problem every winter with my car, which is parked outside to bear the NH winters. It's a cold day, I get inside the car to go somewhere, and there's a frosty layer on all the windows, though especially on the rear window and windshield. Only this ice isn't frozen on the outside, which can easily be scraped off -- it's ice on the inside.
I used to think it was my heater core that was failing, releasing water from the heater ducts. I do not believe this to be the case; I haven't lost a drop of coolant since I replaced my radiator and hoses last year. My theory is that when the car is warm, the moisture that I bring into the vehicle from my snowy boots is evaporated into the warm interior air. When the car is off for a while and the interior temperature reaches equilibrium with the outside sub-freezing temperature, the air is unable to hold the moisture anymore. As such, the moisture condenses on my windows and proceeds to freeze.
Is this correct? Why doesn't this seem to be a problem with other vehicles, such as my parent's VW? Can anything be done to ameliorate the problem? I would use the AC to dehumidify the air while driving, but it doesn't work for a reason I don't care to discover.