Well, after three days of rain, I still have some water dripping in from somewhere, but the volume seems to be decreased by about 90%. There isn't even enough to make it to the back floorboard, but I can feel some dampness in the A pillar padding.

Looks like I get to do some more diagnosis this weekend, but I bet it is a perforation in the hinge area (I could see a 1mm rust spot on the newly black painted hinge area) or a leaky windshield seal like PS says.

later

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84 300TD - 235k - Farbe "Surfblau" bought at 213k
87 300SDL - 131k - Farbe "PimpGold" bought at 115k
00 Klepper Faltboot Expedition Double