...weather? Using my heater when it is not rainy does not cause the windows to fog. My trunk starts to smell musty or moldy during the rainy northwest winter, and the rear window can be foggy in the morning before I start the car, but I cannot find wet places in the trunk. I'm wondering if moisture is getting in around my tail lights, but I don't know where it would collect. The surface underneath the trunk floor and where the spare tire is tends to be dry.

[EagleAutosports]

Because the rain is cold and the drastic change between the hot side and the cold rain causes it to fog up.

[peddler]

As eagle stated, difference in temperature from inside and out causes condensation especially on the windows. It usually does not effect other interior parts and they should remain dry unless you have other problems such as leaving your window down when it rains or holes in your floor boards. Since you mention your trunk floor and spare tends to be dry I doubt if you have anything to worry about.