Ther's a lot of information here about W123 water leaks, but I don't recall reading any about W126 leaks. Anyway, you said in an earlier post that you plan to have the windshield replaced. Do that and make sure they use a new seal. It sounds like you need to have the rear windshield seal done as well. What is disconcerting there is the rust. If there is rust in the channel where the seal goes, the glass shop almost certainly won't fix it. So that you may need to do yourself or maybe consult a body shop. The instructions for rear seal replacement (on a 123) have been posted here more than once. The water in the trunk is probably coming from the rear windshield and not the trunk seal. But if it looks bad and you want to do it, that should be pretty easy.

Oh, and buy your seals from Fastlane using the button at the top of the page because sales there support this site. Somebody here went through that recently and the installer said the seals were better than what his shop uses.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References