At least on my 1990's 124's and 129, when that dang ABS light comes on out of the blue, accompanied by a rough idle, the OVP is the first thing I suspect. I pull out a pre-tested OVP from my "never get stranded" emergency parts kit in a water tight box in my trunk and plug it in.

Do the same and see what happens. The next I suspect is the MAS relay in my cars, in your vintage car, you have three different relays to check and swap good ones in with.

Checking the ground strap is a good thing to do regardless (though unless you are running around without an engine splash tray or live in an area where they salt the roads, probably not the cause).

Make sure to let us know what you found.

__________________
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