Water problem!
 

While vacuuming the E320 out I noticed on the driver's side only in the back and the front carpet was wet. Quite wet. I took the carpet and foam backing out from the front driver's floor. Totally soaked. Took it out to dry, I think the water in the back was actually runoff from the front! The other side of the car is bone dry, even under the floor. I shop vac'ed the water out, and wiped it out. There wasn't standing water on the metal floor, most was absorbed into the foam, but there was a lot. Why?? I see wiring running under the seat and it is wet! Where is the water coming from, and how do I fix it? I checked the pillars, I don't think the water is coming from above. We just had heavy rain, but it could have been there before. I found one rubber drainplug and removed it, but it is in place on the dry passenger side. It is not being tracked in, the car has Mercedes rubber mats in place, and they collect water. Would like to solve this hopefully not serious problem!

I have an older Benz ('85 380SE), and had an even older one ('76 450SE). Both have/had sunroofs. If yours does, sometimes the drain tube(s) can get blocked, causing water to accumulate in the rocker panels and seep under the floor carpeting.

Another possible is the front or rear window seals. If parked on an incline, depending on whether the front or rear is up, the water will 'flow' front to back or back to front. Very frustrating to trouble shoot.

On my old '76, there was a rust hole somewhere on the firewall, behind the fuse box, that always caused water to seep in and soak the front. I gave up finding that one.

Try taking the carpeting up front and back, with a dry floor, and soak the car with a hose. Maybe you'll get lucky and find the source.

Thanks, the sunroof drain problem sounds plausible--I'll check it tonight.