I would think that maybe one of your rear windows isn't going up all the way or maybe they aren't sealed right? Someone suggested washing the inside of the windows with shampoo (not a lot) so they dont fog up. I'm sure there's other things you would wash your windows with too. Try doing it on a day it isn't moist/damp out and the windows are cold. I washed my windows on a cool dry day with windex and they haven't fogged up since.