When it is cold the car is filled with moisture.

Does the air conditioner work? The A/C is used to dry the air before it is directed to the defroster. If you are having high moisture content in the ambient air and no A/C to dry it out, that might be your problem.

Possible heater core leak. It will usually leak out the A/C drain outside the car, depending on the severity of the leak. If your running the heater/defroster and the window fogs up, the moisture is coming from the heater core.

Your heater core is leaking need to replace