minimax

Or you can study the circuit diagram and measure the resistance from socket to sensor, operate each sensor and check the function and measure the supply voltage and earth connections.
Like a proper auto electrician would do.

The system has worked on my car and many others for over twenty years without any attention. Not many mechanisms or electronic assemblies can boast that. 

Stripping the door lock and working the three switches hard for a couple of minutes appears to have cured the problem, probably wiping a bit of tarnish from the contacts.