How many threads are you going to start on this issue? Pick one and keep updating it!

Get a wiring diagram and make your system match it. If it's been hacked up, forget about it working properly unless you were the one who did it and know it worked properly.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)