I would rate a windshield seal replacement as a very difficult DIY job. My experience shows that even a professional can install a new seal such that it will leak. I have installed windshields in other vehicles and it is a real pain. For the fee a glass shop would charge (probably <$100) it's not worth the hassel.

If the seal is not in bad shape...I would take a stab at sealing it with silicone sealant....its cheap and effective.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold