I had a new windshield and seal installed in my 240D about a year ago, and shortly thereafter noted the "old water in the rear floor" syndrome (drivers side). I cleared all of the drains and figured that this had cured my problem, however, it did not help. I noticed during a pretty heavy downpour that I was getting a leak at the driver side corner of the windshield. So I bought some silicone sealer at Advance Auto and carefully sealed about 25% of the windshield at the suspect area.

Problem resolved...no more leaks or water in the floor. Just completed one of the wettest months on record in Alabama (I garage car at night, but park it at work during the day). So....yes windshield leaks can easily be the reason for the "old water in the rear floor" syndrome....and several other reasons that you can find using the search key in this forum.

Good luck.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold