Last night I resolved a problem on my '91 300-SEL that is similar to yours in some respects.
My wife came home the other day after a 30 mile drive back from the big city. The car stalled in front of the mail box. I went out to start it and saw an ABS/SRS/CHECK ENGINE light on while idling.
Got it in the garage and 1st checked the infamous OVP fuse. Looked good - VOM/Ohm check confirmed this.
I became suspicous and tested the battery with car off. It read 11.84 volts and that was on a fully-warmed up engine. I knew right away I had an electrical situation.
Searched the archives hear for ABS/SRS light on issues. Found numerous items. Most pointed to a faulty regulator brushes. Put the front end up on ramps and pulled the regulator out of the back of the alternator. Part number 1-197-311-023 is the
regulator. The outer brush was about 1/16" long; the inner brush was about 3/16" long. Bought a new regulator. The brushes in the new regulator are about 1/2" long. Installed regulator, reconnected battery cables(had to rechange battery some) and the ABS/SRS/CHECK ENGINE lights are now gone once started. When I first turn on the key, I now get the various dash lights. When I had only the ABS/SRS/CHECK ENGINE light, I would see none of the others light up with the ignition was first switched on.
Check your voltage regulator. It's worth a shot.