This sounds like the central locking 'attenuator'. It looks like a fuse box relay which is about 30mm cube, but it is three times longer, ie 90mm long.
If the layout is the same as the 'R' reg that I had, it is mounted behind the driver's door panel, below the wing mirror. The unit just clips into place.
The alternative problem is the door actuator in the rear door itself, which may have broken away.
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