Masa'warty 3200... Talk To Me!
There is dampness in the car.
Can be for loads of reasons. This time of year it rains more so you would be getting in to the car damp and the dampness just gets into the car. You may have a leak somewhere into the cabin. Check that all your carpets and stuff are dry. As you say its the rear window check that there are no leaks in the boot, that maybe getting up through.
If you can get your hands on some Silica Gel packs and leave them in your car to see if that helps the problem. Note not the wee tiny ones you get with a CD player or something would need to be the size of your wallet or bigger. Check online there should be places that sell them.