The trunk in both my 300D's also leaks, but I don't see any rust-thrus in the gutters and strive to keep them clean. There isn't much slope to make the water run aft, which doesn't help. But, too much rain is the least of our concerns in CA. I have a black plastic liner to keep the carpet clean anyway, so just a few drops get on it.

You can get a long-handled "gasket scraper" that holds a single-edge razor blade at HF and elsewhere. If you don't tear and mangle it, 3M weather-strip adhesive (yellow, any auto parts) should re-secure it.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans