Just last month I solved this symptom on a 240. I don't know that the 240 works the same as the 740, but in the 240's case the central locking would lock the trunk OK, but not unlock it. That it locks absolves all of the electrical given the unlocking occurs in the remaining 3 doors.

The fix was lubricating the lock part of the latch so the lock motor could overcome the friction and dried up grease that had developed over time. Little dab of that lithium grease...

Now if yours neither locks nor unlocks, then you might suspect electrical trouble, or as often happens in the doors, water gets into the lock motor.

Vacuum sounds like Mercedes-Benz...
--
Art Benstein near Baltimore

A good wife always forgives her husband when she's wrong.-Rodney Dangerfield

If your problem is corrosion - our 88 745 GLE had the flap that covers the key hole allow it to get badly corroded inside.

I removed the inner panel from the tailgate and removed the lock.

Then I cleaned all the parts and discarded the lock cylinder - the electrical portion worked after the bad parts were gone,

I then attached metal disc (aka - a penny) over the key slot.

I've recently developed the same problem, my tailgate no longer locks when I lock the car. The lock cylinder was corroded beyond repair when I bought the car 5 years ago, so that's not an option. What this means is I can no longer lock the cat against entry from the tailgate.
--
1992 745, >500k km

I'll ask the obvious. Are you sure you are not in the "valet" mode?
--
Post Back. That's whats makes this forum work.

It's not vacuum, it's an electric solenoid. It could be that the rod to the lock has become disconnected, have someone listen while you turn the key. Or the solenoid has failed, but I think that would make the key not functional as the solenoid would not allow the key to turn.