Thanks Dieselbenz1, but I'll be happy to just have a manual-working mechanism working again, if I can tame the little critter.
When I gaze inside, I can see a vacuum arrangement of some sort, that seems disconnected from the mechanism, so looks like someone gave up on it in the past, best to let sleepy dogs lie...

So my plan is to remove the assembly from the door and check out its operation on a bench, which would probably make a lot more sense than trying to figure out what linkage does what inside the door. A shame that MB didn't deem it necessary to better document this contraption in the service manual. The manual limits itself to showing how to remove it from the vehicle, and how to put (a replacement one?) back in.

Onward ho.

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1983 300TD 240K - 1982 240D 215K - 1996 Dodge Cummins 70K