Need Automotive help: Pontiac Montana front window defrost problem.

We own a 2002 Pontiac Montana mini van. Last winter, in upstate NY, when the weather was below freezing the defrost would not clear the entire front window. I took the van to the mechanic and they cleaned the cabin air filter. That did not work so I took it too a different mechanic who said the carpeting was wet and the fan was sucking up the moisture and told us to dry out the carpet. It still did not fix the problem. What exactly happens is the window defrost will sound like it is blowing, it is blowing some but the window still fogs and below freezing the window actually freezes to where you can not see out. I really would like to get this fixed before this winter.