I have the SAME problem in the 400E. When I go through the automatic high pressure car wash or when it rains hard, it soaks the passenger floorboard. The problem (at least on my car) is the waterproof boot that surrounds a couple of wiring harnesses that go from the engine compartment to the cabin through the firewall has cracked. On my car it is the uppermost wiring harness on the firewall on the passenger side (way up in the corner.) Look at yours and see if the boot is cracked too.
Hope this helps yall....Peter