Inside the drivers door theres a trigger switch that is attatched to the lock mechanism , its the one that sends the signal to the locking interface(certain models/not sure on yours) . its easy to find as its the only wires that go to that specific part.
We had 2 or 3 breakdown which caused the central locking to instantly unlock itself when you tried to lock the doors. its expensive from a mainstealer and maybee cheaper to fit an aftermarket remote locking system if its buggered.