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

__________________
John
1986 300sdl
1985 380se