Oldfart
FRF Addict
I don't know for the OP, but like Nex said, check your rear cab vents. There is one on either side of the rear cab wall a few inches above floor level. It's just a gravity controlled flap to relieve pressure when you close your doors. Not too hard for one of them to get stuck open.I had a problem where the truck was building up moisture underneath the rear seats when it rained hard. Might want to check and see if there’s any moisture there as well. I haven’t been driving my truck a lot either and put a box of baking soda under the driver seat and that seemed to help a little as well.