Moisture frozen on the inside is a result of warm moisture in the car prior to turning the vehicle off for the night.If you run the heat on the floor often then moisture from snow, mats and damp carpets evaporates and clings to the inside of the vehicle at night.During operation of the vehicle make sure you run the heating system on "recirculate" to dry in dry air from outside thru the heating system and not recycle the air from inside the vehicle.At night, lower one window just a bit to allow moisture to escape.Check your owners manual for insight.