I own a SL600'93. The car was running with low power issues that pointed the cabling could be the problem. Found both ETAs bad. Rewired right one, no problems. But after rewiring left one, it starts rough, ASR light goes on, and rpm does not work. Checked all the wiring in the ETA, but I can't find anything wrong. I am pretty sure it runs in all 12 cyl, but didn't keep it idling for long.
There is a diagram that I made some years ago here, you can at least check to see if we have the same mapping:
MB SL600 7,2 Všth