In addition to perhaps trying auto silicone -- which I used successfully to seal a leaking Jetta (diesel, of course) windshield - was still holding 4 yrs later, if not the prettiest.......... you also might want to pull your carpets up & check the rubber floor plugs -- my 240 had 2 that were missing for some reason ---- the dealer was no help -- spent $6 on plugs he warned me were non-returnable because they were special order & they fit nothing -- found a plug that was close & a rubber cork that I seated with a hammer - both at a hardware store -- sealed both in with GOOP -- can't tell with the carpets back in & not a drop of leak.
Ed