It is probably your drains from the A/C. On my 86 300sdl, I had a wet floorboard on the drivers side. When I pulled everything out on the drivers side you could see water had been under there for a while. I dryed everything up and looked at the drains coming from the A/C unit. There are two, one on each side of the center tunnel. Mercedes used a foam tube with a wire coil in it to keep it from collapsing. The foam gets old and brittle and it literally crumbled in my fingers. I pulled all the old stuff out and check on new drain hoses from a dealer. They are still made of the same foam and wire coils. I decided to use some rubber hose I had laying around. It actually was a sunroof drain hose from a 1990 Volvo 740. It fit perfectly and had the consistent rigidity to withstand the bend it makes going through the transmission tunnel. I cut it a little long so it fits down on either side of the tranny and ends below the bottom of the tranny. It has been one year since I did this and it is working great! All of the condensate from the evaporator goes right out the hoses and onto the ground, not onto the floorboard. Be patient when you take all of this apart. The plastic pieces for the rear seat heat and all of the little parts get fragile.
John
86300sdl in hot, humid, NE Louisiana