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