I had what maybe the same problem with my 87 300D. I rigged a ignition switch bi-pass, but only for the starter, the ignition/acc, part worked, only the start possition didn't.

Oddly, the ignition start position works again, but I still have the bi-pass switch installed just in case. Before I installed the switch, I used a wrench on the starter to jump the wires to start the engine.

Good luck.............