Thank you oldtrucker for the advise and insight. I had an idea the monitor would read the rated current through a circuit to know is something was amiss. That's what has me stumped now with the rear side marker lights still out why the bulb failure light doesn't come on. It would only activate when all lights (headlight and running/park/) were on. I thought the dim headlight might the cause but apparently not. After reflowing those pins I found it doesn't come on.

Looking at the circuit diagram the stop/brake lights go through the lamp monitor. Whoever found the brake light circuit dead at the lights bypassed the wiring harness just for power and not ground. If there was an issue with the lamp monitor there wouldn't be power to the brake switch correct. Looking at the flow the circuit runs from the brake light switch through the lamp monitor and out to the lights. I need to locate the X21 junction which includes connection to cruise control and ABS.

The other question was the interchange between models for the lamp monitor itself.

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1983 300SD
1988 420SEL