In my '79 (not sure if it's the same in other years), you have both wires going to the motor getting 12v when the key is on. My driver side window wasn't working and I found that I had no ground at the switch.

Again, not sure how later 240s are wired, but IIRC I had the hot wire to the window switch that provided power to the entire switch, and sent constant 12v to both sides of the motors. I was missing a ground on my driver side switch, but the rear window still worked because it grounded out from the window switch in the back door.

Where are you measuring voltage?

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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