I think there is a thread on it somehwere, there is a relay that triggers the door locks but sometimes the connections in the doors get exposed to moisture and can corrode or they break through vibration or movement - I am sure I have seen somewhere a broken wire close to where it goes through the door into the body of the car.
It means pulling your interior door panel off to have a look at the wiring and connectors, you can also test to see if a signal is being sent when you use your key fob (a 12v bulb and some wire, put one -ve wire to earth and put the positive on the signal wire, press your key fob if the light flases it is getting the 12v signal to activate the relay)