Follow up: I did a test on the driver's side door. The motor works fine with voltage applied directly. Inside the door was a bit wet for some reason, and some wires at the bottom were looking pretty corroded. are those the central locking control wires?

voltmeter tests at the terminals show nothing, no voltage going to those terminals. All I can think of is it's either the wiring ( but in all 4 doors at the same time?) or the relay unit... anyone else have any ideas?

thanks!

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1967 250SeC
1979 450sel 6.9 (usa)
2009 S550