Trunk lock problem-W126 300SD

[ytr1903]

I have a relatively small, but irritating problem with locking and unlocking the trunk lid on my 84 300SD. I have searched the site and I have looked at my repair CD, but the solution still is not in sight. The key operates the trunk lock mechanism correctly. Counterclockwise locks, clockwise unlocks. At first, I thought that I may have only valet keys, but I have watched it operate with the trunk lid open and can see the proper movements of the latch. The central locking system works perfectly from the driver's side door location and properly locks and unlocks everything including the trunk. But when I try to unlock the trunk with the key after having locked the system at the door, the trunk will not unlock, nor does any of the other locations unlock. I thought at first that there might be some sort of electrical switch at the trunk that actuates the central system, but the CD diagram and my visual observation only reveals a vacuum diaphragm at the trunk lock. Any thoughts or solutions?

Tom