I´m not sure if this is a common issue but I think that everyone should look under the seal that is under the backglass. I have some rust there that I´m going to take off. I sprayed the “lip” where the sealing goes with some corrosive prevention oil so that it won´t get worse.
I had the same thing on mine. in both corners of the rear window and up in the high corners as well under the window trim. since I was doing a lot of other things and the rust was really bad (rain water came through)I took the rear window out and welded in new metal.
but if its not that bad perhaps some wire brushing and some rust remover followed by primer would sort it out for you.
I haven´t looked under the window trim in the high corners but I hope that they are not rusted. The rust under the seal under the glass has eaten up some of the “lip” where the sealing strip goes. I´m going to take off the rust with rust remover and put some chemical metal in the lip. I don´t own any welding equipment but I think that I can repair the lip without them.