Does your A/C work? A/C is used with the defrost as well as when you first start the car when cold out (in Auto mode). Try to stay away from using Econ and Recirc for a while.
The main issue is moisture in the car. Check all the carpets for signs of water (not caused by tracking in snow, etc). I had a plug missing from one of the frame rails under the car causing water to enter through the floor.
I also would get water in the trunk from the sunroof drain hoses being out of place and poor sealing taillight gaskets.
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