On W123's the Cruse Control picks up it's ground thru the Brake Lights......at least 1 of the Brake Light filaments must be intact to enable the Cruse Control....power to the Brake Lights causes the Cruise to drop out (electrically canceling the ground)......

I don't have any schematics for your car, but knowing the Germans, if it is working soundly, incorporate it in the next design......so you might look at this section of the proper schematic.......

at least that is my understanding of how it works.....on a W123....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG