The C/L relay operates all the lock motors together so if the rest work OK it can`t be the relay.
The tailgate or boot should still unlock manually with the key, if it doesn`t you have a mechanical problem with the lock which is preventing the lock motor from working, if it is stiff it`s probably seized, or the linkage is disconnected.
If it works OK with the key you either have a faulty lock motor or the wiring to it is faulty.
The most likely wiring fault is a broken wire between the tailgate and the body caused by the flexing as it opens and shuts.
It sounds as if your "mechanic" doesn`t know what he is doing and will just throw in expensive parts until the problem goes away.
My advice is to either do it yourself or take to someone who knows how to do a proper diagnosis.
Colin.