OK, so there is an electrical switch that is used in these doors to activate lock/unlock? I do see one wire that goes up close by the lock mechanism, so maybe that will lead me to the switch. Hopefully, just switch can be replaced.

The reason I didn't think there is an electrical switch is that at the trunk lock I saw no electrical wires, only the vacuum line. And when I lock at the trunk, it locks all other doors just fine. So I am baffled at how that works if there is not an electrical switch. I think the older models used a three-hose vaccum switch, but maybe my 85 300SD does not.