Living in the land of ice and snow we fight this kind of problem too often. Lumbergh is on the right track. It's the difference in temperature from the inside of the cab and outside the cab. Think of all the moisture held in the cab (wet rug, humidity, breath, etc.)then touching the cold windshield. It condenses and there's the fogged window. It may happen to the dash or anywhere else that's colder than the moisture laden air. We may not notice it elsewhere as much because we're trying to see through the window. The window works as a condensor.
I have been told to run cold air on a window and it won't fog as much. When you put the defroster on your new vehicles the air conditioning comes on to clear the moisture. Of course it is cold and dehumidified. I don't know if this makes sense but it does make a difference in my vehicles.
Rain-X is great.
Learned this trick from one of my students - Try waxing your windows for winter and the ice and snow comes off much easier. Don't know about waxing inside for defrosting.
Rick
