On my W124 the overflow bottle is in the quarter panel. I suspect it is the same for you. Either the overflow bottle is full, or it is damaged. You need to remove the plastic inner wheel well cover to get to the bottle.

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.