This is a very common problem on older Mercedes as you know. My 85' had water in the passenger rear floor as well. I was told several times that it was probably the drains under the front cowl. I checked them and they were clear. I actually noticed that my water problem went away after I changed my antenna seal - which is odd, because the antenna is on the driver side and the water was on the passenger side. Strange, but it went away after that.

It was a week later, I thought I would check the drains anyway, as all of the water had not evaporated by then - but as mentioned, they were clear. Since you have a wagon and the floor setup is different, this may not be your problem. It may actually be your drains.

Good luck

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2004 Toyota Sequoia Limited 4wd
1991 Lincoln Town Car Executive
1991 Cadillac Sedan DeVille
1988 Mercedes 300SEL
1972 Chevrolet Caprice Kingswood Estate 9-passenger wagon
1973 Pontiac Grand Ville
(Prior MB's: 1974 240D, 1985 380SE, 1984 190D, 1993 400SEL)