MB issued a Service Campaign in March 2001 instructing dealers to check and clean the reed valve at the bottom of the air duct/water collector. I don't know why, but this procedure was never added to the list of items to be inspected during regular maintenance.

Nearly every time we get heavy rain, I get a 220 S-Class or two towed in. The reed valve at the bottom of the water collector gets clogged and the level eventually rises until it reaches the fresh air intake. It then rains down past the cabin filters and blower motor, onto the passenger front floor. If enough water gets in, it soaks through the floor carpet and works it's way toward the rear SAM/fuse box. This can cause a wide variety of problems from the engine not starting, to various rear lights staying on. I had one a few years ago that actually melted the wiring and fuse box due to a shorted rear defroster relay.

lizem100, I'd strongly recommend removing the rear seat bench, and checking for water in this area. It only takes a few minutes to check, and you could wind up saving yourself a major headache. In a worst-case scenario, the typical parts needed are the cabin filters, blower motor, rear SAM, and rear fuse box. The tow-away sensor might also get wet, but that takes lots of water. The carpets will also have to be removed to dry them out. Cosidering that it's an S-Class, the $$$ damage isn't as bad as you'd think. Hopefully, you won't find any water, but if you do let me know.

There are two simple ways for S-Class owners to avoid this potential disaster. First, check the reed valve regularly. If you live in an area where the seasons change, check at least once a week once the leaves start falling. All it takes is a flashlight and some stiff mechanics wire. It takes all of three minutes.

An even better safeguard is to drill a few small holes in the side of the duct, below the level of the air intake. By doing this, even if the reed valve does become clogged the water will leak out of the duct before it reaches the fresh-air intake. There are work for instructions detailing the MB approved procedure in WIS.