The circular duct you are refeering to is the heat duct for the rear passenger's feet. It goes up under the seat. If you slide the passenger seat forward, you will see the outlet for the heat.
I had a similar water leak in my 1986 300sdl. It turned out to be the foam hose that drains your evaporator core for your air conditioning. On the front transmission tunnel there are two drain hoses, one on the driver's side and one on the passenger's side. They attach to the drain pan for the evaporator and then drain through a hole in the floorboard.. The original hoses are made of foam with a wire coil in it to keep it from collapsing. Over the years the foam disintegrates and they leak water when you have the A/C on. The water then travels along the transmission hump and pools under the rear carpeting. The reacement hoses are identical in construction. I did not like them and replaced them with rubber hoses that will not deteriorate.
I would look under the front carpets on both sides and check out those hoses.

Good Luck

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John
1986 300sdl
1985 380se

I would also remove the battery and the battery tray. Vacuum all the junk out and you may find a rust hole under all of the junk under the battery. That would allow water to drain right into the area in the passenger front footwell, and then it can pool under the seat.

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John
1986 300sdl
1985 380se

Brandj,
Thanks for the tips. Stupid question but what if the AC in my car does not work. Would I get the same symptoms when it rains.

The localized area where it gets wet is the area closest to the hump by the rear mount for the passenger seat.

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Ben
1987 190d 2.5Turbo

That's a good question. I don't know if water could get into the plastic case that the evaporator and heater core sit in. If it did get in there from the air inlet by the windshield, I would think it would leak out of the blower motor housing. I would still look at those hoses and where they exit through the floorpan. Have you been driving through deep water?
I would still look under your battery and battery tray to see if you have a hole into the car from some battery acid. Heck, it could even be your windshield seal. Water leaks are tough to find, because they will follow the easiest path and then pool in the lowest place in your car.

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John
1986 300sdl
1985 380se

There lies the dilemma. I will attempt to find these drain holes that you speak of. Are you saying that it is NOT the black rainbow shaped thing under the front passenger mat? what is the diameter of the hose you speak of? what color? thanks! this is really helpful.

by the way, the drain under the battery pan is clear. that was last month's project. don't you just love these old cars...they keep you busy!

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Ben
1987 190d 2.5Turbo

The original drain hoses on each side of the transmission tunnel were a gray color. On the driver;s side it is next to the accelerator pedal. On the passenger side at the same place. They are just drain hoses, that come out of the bottom of the heater/evaporator box and go directly into a hole in the side of the transmission tunnel. If they are original, they are very fragile. Mine disintegrated when I touched it.
I believe there have been some threads on here about them. Try searching AC drain hose

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John
1986 300sdl
1985 380se