In my 2006 Grand Caravan, after it's been raining, I depress my rear defog and the radio, time and fan cuts in and out. The lights in the Console go dim to bright. When I depress again to turn off rear defrost, all goes to normal. Once it's been dry a day or two, it stops doing this and the rear window clears up. I suspect a leak in my back door where the cable comes in at the top.
I'm trying to find schematics for the rear defrost and also a diagram on how to remove the rear door.