I just ordered a non hardening Windshield Sealant from Fastlane. $22.50 plus shipping. It comes from Mercedes so you may be able to get it at a dealership. I have not tried it yet. I did use a hardening type on my windshield and it helped a little but did not solve my problem. I do not know if there is enough room to get a sealant in the rear window under the gasket. See if you can lift up the gasket at all. If there is room to squeeze some sealant in there, you don't have a lot to lose by trying.

I am sure someone with more experience will chime in and give you some direction.

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