Rear door has a solenoid to push, and pull the lock mechanism.
Firstly try the fuse named "Dome Lock" under the plastics drivers side. This controls rear lock and one or two other things.
Secondly if that doesnt work it would be worth getting a replacement, from a scrappy. As solenoids either work or they dont. I may have one lying around, if i have i`ll pm you.
Although its worth checking that fuse and the connectors in the rear door.