2002 Nissan S-Cargo central locking not working

Check fuse first,if not that then remove dr panel and check that nothing's popped off ,could be a rod off the solenoid,if all okay mechanically then you would have to check the wiring probably switches to earth when solenoid is active.

I took off the inner door card today - disconnected the wiring plug from a white plastic unit - ascertained which was the live feed - it has three other wires and I assumed the black was the earth / ground -then by using a paper clip from the live feed I contacted the remaining two wires and was able to lock and unlock the other five door's electronically - the rod comong from the barrell moves a lever which is attached to the white plastic unit. It looks quite awkard to disconnect and remove this white plastic unit. Should I try to take this white plastic unit off / out and open it to try to fix or perhaps clean the contact that it should be making?

Thanks for the help.