So on my 91 300D I discovered that the front passenger floor and rear right carpet were soaked. Removed the front "floor" (the piece with the foam rubber).

Sent to my mechanic and asked him to confirm drains were clear and to diagnose issue.

He said drains are clear and the area the looked suspicious to him was the area above the rear windshield: when water was sprayed on it would disappear through top...it looks as though gasket is missing. But he said there was not much more he could do without removing panels, etc...

So to test the rear window hypothesis, I taped it off and waited for it to rain.

With the car slightly nose down parked on the street and a steady rain falling, I noticed the following: water is coming up through cracks in the floor. So I'm thinking maybe water is entering through the engine compartment and there's some sort of breach in the firewall.

But then, later that same day, now with the car parked in the driveway and no longer nose down, we had this horrifically torrential rain. I was certain that the footwell area would be soaked. Instead, it was bone dry. The only difference in the two scenarios is a slight difference in the angle of the car.

So now I'm suspecting that the intrusion must be in the rear....to account for what I've observed.

Grateful for any insights as to how further to investigate.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife