I had a problem with this. Check all your door seals and check for gaps. I had a big one on both sides of my rear doors in the curved on the back part of the windows. Did the same thing as what you're talking about. Check for gaps in the seal itself and gaps between the seal and the door frame (which was where my main problem was). Hopefully a board vet (randman, do you hear me?) will come in and give you advice on how to actually FIX it. I fixed mine, with my dad'* help, but I suck when it comes to giving those kinds of instruction, and I don't want to mess you up
Also, I may be missing another possible cause, but a leak like the one I told you about is a definite possibility. Hope that helps
And one more thing - welcome to the BCF!
*Resident 'Rip Van Winkle' of the Bonneville Club*
"I want to know God'* thoughts; the rest are details." - Albert Einstein
"Not everything that counts can be counted, and not everything that can be counted counts" - Sign hanging in Einstein'* office at Princeton