I've got the same issue with my wagon, plus I have water getting inside the rear-most third seat foot well. Water is getting into the front passenger foot well area, so I've pulled the carpet out.

Regarding the hatch, my car has bodywork (bondo) at the right rear corner, so I suspect the hatch doesn't completely seal as the body is tweaked or the hatch is tweaked. I have not yet tried to fix it.

The water getting into the forward part of the car is also a mystery, I am suspicious of the sunroof drains (there are four) but I have not yet definitively determined that.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Well Max, if you look at my thread history, you'll find one where I went through an exhaustive process to eventually find water was actually seeping in from a small crack in the front passenger footwell floor. The water was collecting because of the ridiculous amount of blockage that I had on the front passenger side drain. To access this one, you'll need to remove a few screws on the fender liner etc. I took the tire off for better access.

For the rear, mine has no bondo and no water (so far) in the 3rd row footwell. I have no doubt that If I didn't open it to check, that depression would overflow into it though. I wonder if its coming in through the hatch seal, flowing INSIDE the hatch and following the motors cables or whatnot into the lock depression.

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1987 300TD
1984 300D
755,000 KM and going strong
BC Canada