You didn't say if this is in the front or rear. I have had more than one 123 sedan that needed a bilge pump in the rear floorboards. I puzzled over it for years and finally got the answer when I found mercedesshop.
There is a cavity at the rear corner of the engine compartment that the hood hinges fold into. Clean the cavity and clean out the drain with baling wire, sorry I'm a country boy, I'll bet a coat hanger will substitute for baling wire. I cleaned out these drains and have driven in driving rain since then without having to use the bilge pump.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in