Date registered: Feb 2008
Vehicle: 1987 420SEL
Location: Ontario, Canada
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I had the exact same issue with my 300D, and it is an inherent problem with most LHD W123's.
The super-hard plastic filler you describe is just a type of sealant used by MB (and other manufacturers) to fill in seams and valleys during the manufacturing process of the body shell. If you remove the plastic surround on the passenger side of the cowl, the little black (on lesser models, chrome on 300's etc) grate, you will find the drainage areas for water that comes into the grates. To remove, simply push the centre section of the square plastic rivets in, and then pull them out. Be sure to retrieve all of them, or purchase a new set from MB. You will probably have to remove the windshield wiper arm for that side too.
If these areas are leaking, dry them out thoroughly, preferably over a few days (no rain, heat gun works but careful not to burn anything), and seal them with whatever quality sealant you see fit. Home latex caulking is no good; you'll find a satisfactory product at your local auto parts store.
There IS a way of re-routing the water to flow away from the passenger side vent (inside), by attaching a length of rubber hose to route the water towards the left side of the inner cowl, but I haven't seen the inside of the cowl for a W123 in a long time, so I can't provide specifics. There is a way to do it, however.
(On Edit)- I'm pretty sure the rivets only go in "one way", as they're not entirely square, so mind that.
Last edited by Benz Prodigy; 01-17-2009 at 12:41 PM.