I would not rule out the antenna seal (if the coupe has the antenna on the back like the sedan). I had a 1985 380SE sedan. When it would rain, the trunk and believe it or not, the right rear passenger floor would pool up with water. Upon someones recommendation, I replaced the antenna seal and the water never came back. My floor was as dry as a desert in the middle of summer. Trunk was dry as well. I have another W126 now and I am getting a little water in the trunk, but again, I think its the antenna seal, because there is NO rust around the rear window.