I just went through the same aggravation.I repaired and replaced 3 different central locking "rings".As soon as I plug the thing in inside the door,the locks would lock but not unlock.Drove me nuts.Finally I unplug the wireing inside the car,mine is a 92 240GL,pulled the harness out a little from the door jamb,rubber boot and fiddle with wires a bit.And the locks worked.The wires in the door were not cut or frayed.But I suspect where they 'bend'it caused an internal disconnect.I wrapped the wires with tape to make them strong put it back together,made sure the plug under the dash was not corroded and everything is now working the way it should.I would trace the harness from under the dash ,through the door and out to the plug.I was lazy,otherwise I would have cut the end plus off and run 3 new wires.More work but a better repair.