a couple of options
I am research the same issue. Many folks say the rear window seal goes bad. I bought one, went to get it replaced, and the auto glass place said it was the trunk seal - he went in the truck with a flashlight and had someone pour water in different spots. (Let me know if you need a window seal, it is a Mercedes special order part they wouldn't take back).
I think I may actually have a bit of rust in the driver corner of the fender near the base of the rear window. Time to try and seal that.