GSXR is correct -- 90% probability it is the combination relay.
Pull the fuse box top off, and then remove the rear cover over the relays. This is held on by four Philips head screws, and is best/easiest removed with the hood up in the vertical position.
The Combo relay is the second largest relay in the relay box -- a rectangular black relay that should be on the left side as you are facing the engine/windshield from the front of the car. (The largest/longest relay in the box, on the right side, is the lamp burnout indicator relay).
The MB part number (which if original is marked on the side of the relay) is
201 821 02 47. This is an extremely easy to find relay at wrecking yards, and was used in most W124 models Ive seen, as well as Gen 2 (1986-1991) W126 sedans and coupes, and many W201 models as well. I keep 4-5 extras of this relay -- over the years Ive had them go out in two of the four W124 cars Ive owned.
Sometimes you see these have been replaced with aftermarket units (often KAE) which are 90% of the time, utter junk that lasts a year or two before failing. Plus, they want like $65 for a Kaehler. Forget that! Best to just get a used unit or two (easy to find on eBay or wrecking yard, as I mentioned) and slap it in there.
eBay example
is here.
Cheers,
Gerry
OE MB relay
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Junk Kaehler aftermarket relay
View attachment 35295