The water shouldn't be running in, the seal should stop it. If not, either the seal is bad or the lid is not properly installed/adjusted. Has the car ever been hit in the rear and repaired?

Otherwise, the tail-lamps can be the problem, I had that in my 201 (similar setup) and have removed/re-sealed the lights in all of my 124s since just to be sure.

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Gone to the dark side

- Jeff