Are you measuring voltage across the 2 terminals at the door connection? Those should both be 12v hot when the key is on, so measuring across those two will net you a 0v reading. You should ground out your voltmeter to a known ground, like the door check strap or the door latch.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission - engine spun a rod bearing, sold to pick your part

1979 300SD, ~90k original miles, all stock