All good suggestions. I'm glad I asked.

I have looked at some wiring diagrams but my ability to read and understand some of the technical items is limited. I replaced the combination switch 3 years ago with a used one. When I bought the car, the turn signals were inop and I diagnosed that to be the stalk switch. That doesn't eliminate the possibility that the used stalk is bad. I have a spare of unknown condition I can plug in for testing.

The ignition switch is something I have not looked into. I have some spare key, lock and switch assemblies from gas W126 cars. I wonder if one of those would plug into the harness on the 300SD for testing.

I was also thinking about taking a jumper cable and connecting it the negative battery terminal and somewhere on the engine to act as another path to ground. If my issues magically disappear, then I will know it is ground related. I might try this first before pulling electrical wires.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge