I just did this on my W126, it was a pita of a job but not that bad. Just follow the advice above and take your time.

Use only an OE seal as well the aftermarket ones are junk. You will also need some sort of window sealent. I used the MB stuff.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200