REPOST FROM REDUNDANT THREAD
I have been doing this stuff for 30 years and my first step would be to get out the wiring diagram. Do you have the 124 CD? In it will be the ETM, electrical troubleshooting manual. It will show the theoretical layout of the system and show each connection or component involved. It will also show the location of where they are located on the auto.
You can't do this kind of work without that tool.
Bosch Master, ASE Master, L1
27 years MB technician