You likely have moisture getting in the car somewhere.
Check the floor carpet for dampness.
Search sunroof drain (assuming you have a sunroof) and door vapor barrier.
If you have a 4 door car it is HIGHLY likely your rear door vapor barrier(s) is(are) leaking. The rear door vapor barriers AUTOMATICALLY leak after 6-7 years.
The front door vapor barriers can leak, but not as often.
Also if you have had any work inside the doors, this is a likely place to check for leaks as well.