You could also check the bottom of the rear wheel well. This is an area which leaks on mine, especially when driving. there is a place at the bottom that rusts out and there is also a body plug higher up that rusts around the perimeter. I would say the body plug is too high up to leak when parked. You could also remove the fender to check that area. There is a spot behind the fender close to the door that rusts out and can leak.

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1990 300d 2.5 turbo
1995 E300d