David,
I agree with the others on the right door lock.. sounds like a vacuum issue. In addition to possibilities already mentioned, could be the vacuum actuator in the door.
On the rear head rest, I think it depends on the year.... my '96 W140 Coupe rear head rests are electrically driven, not vacuum. Believe early years were vacuum driven.
Does the head rest go up and down with the console mounted switch? Is it just the switch in the rear console that doesn't work?

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KenP

1996 S500 W140 Coupe