Wow, this has been beat to death here. There are hundreds of posts regarding this exact problem. A search will yield the info you seek. You'll find that the culprit is usually 1) The hinge pocket drains 2) The Battery Tray 3) Windshield Seals or 4) the Sunroof Drains in that order. It is a time consuming easy fix though.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick