(I just replaced mine).

It's called a "multi-function relay" and it's in the compartment just behind the fuse panel. Take off the fuse cover. Raise the hood to full vertical. Unscrew the relay panel cover (just a few turns, the bolts have a backing that keeps them with the cover even though they have been freed from the base. There are also two bolts at the front of the row of fuses closest to the front of the car). The largest black box at the middle of the set of relays is the the one! You just wiggle it up and out while holding down the base of the relay panel.

About $84 here. $30-40 at a MBZ recycle/salvage yard near you I'm sure. It seems to be the same part for any of the 124's.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio