When I was a VW tech in Seattle this was a constant problem, especially with the A2 chassis cars. The vapor barrier, which is the plastic sheet between the interior door panel and the door itself would get a hole, or start to peel away from the door and allow rainwater to fill up the floor with water. I don't know the 126 well enough to know if this is a problem with these cars, but if nothing else works....
It also did a good job of turning the bottom part of the interior door panel to mush.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)