You answered my questions. I like the idea of having a connector similar to the ones already there, with three pins and just using one or two wires as necessary. So, HOT + GND or IGN + GND for a typical device. I figure you can find ground anywhere on the chassis but this is about as good as it gets!

Did you say you can get these 3 pin connectors on the back of the instrument cluster only?

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1990 300E
124.030
chassis 103.983