I have an 88 300e. Whenever it rains, I get heavy condensation on the side and rear windows . I do not use the recycle air button ever. The rear window can be cleared using the rear defogger button. The front side windows but not the rear side windows can be cleared using the heater.

When it is cold, I get frost on the inside windows. The rear window can be cleared using the rear defogger button. The front side windows can be cleared using the heater.

These fogging/frosting problems appear when I am driving and when the car sits unoccupied.

Any ideas where the moisture is coming from and/or how to get rid of this problem?

Thanks.