You need an Electrical Troubleshooting Manual. We sell them on the CD set for the 123. (hint. the CD is broken into 2. One is for gas and the other is for Diesel.)
You may try to split the cost with someone.
You have a ground problem somewhere.
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM