I Live Above The 45th Parallel. Iknow What Frost And Cold Are, Too.
Let me guess that you make short trips - 8,000 miles in one year - That's about a fourth of what our cars go in a year.
The car is moist inside. Press your hand into the carpet in all four footwell locations and make sure it's not outright wet, indicating a possible body leak.
Besides all the above comments, the car needs to be dried out. If you've got rubber mats covering the carpet, temporarily remove them and take a road trip or several road trips with the heat selector set to "floor" and fan on full blast. You may have to open a window slightly (or lots) and remove your jacket so that you don't personally overheat.
Ideally, the car should have been parked with all doors open for awhile this past summer on a sunny day, but that ship has sailed.
You could see if there's a kind person in a local body shop that would be willing to "bake" your car in a paint booth for you, but you'll just need to plan on making some longer drives with floor heat at least once per week to maintain dryness.