I am starting to believe that there may be multiple potential failure modes with this multi-function relay/buzzer.

This is based upon a search of posts, the wiring diagrams, and the fact that my silly seatbelt warning light stays on indefinitely if the passenger door is opened at some unknown elapsed time after the ignition has been turned off. (Not right away!).

Anyways, all roads (errr, wires) point to the relay/buzzer.
I'll report back my results when I get to it.

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1986 300E 5-Speed 240k mi.