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What I did on my 95 with the same issue was to slice it exactly at the corner seam. Then the vertical and horizontal pieces could be put back in their channels. Then I just filled in the corner (since the shrunken vertical and horizontal pieces don't quite touch anymore) with the black window seal adhesive stuff, which I also used to re-attach the two separated sections into their channels. It doesn't look great, but it looks a lot better than it did, and more importantly to me it doesn't leak after many years. It's quick, cheap, and easy, and if it doesn't look good enough you can still always have the whole thing replaced.