The cig lighter (the one on the front left) is on its own 10A fuse (fuse 53)
The front and rear power outlets share a 15A fuse (fuse 42)
If both the front & rear power sockets have failed, it could be the fuse or the relay - fuse most likely.
If the front power socket works, that means the fuse and relay must be OK, so its either the wiring to the socket at the back, or the socket itself. In this case, unplug the rear socket and put a voltmeter across the wires to check you get 12V. If you get voltage, the socket must be knackered (might just need a clean, these type of sockets never give great connections), otherwise suspect the wire.