Maybe this changed with the later phase 2 N16s but on my phase 1 it explicitly states that the rear 12v socket is not to be used for cigarette lighters (I guess it and it's fittings are not heat proof) and the little tray below it it not suitable for use as an ashtray (again, not heat proof). Maybe something melted if you are using it for a lighter?
If you just mean the 12v socket is not working and you've not been using it for lighting fags (I use it to charge phones, BT headsets and my laptop) then check fuse no 3. It's on a different fuse because the rear socket is powered directly from the battery while the front socket for lighter is on a circuit behind the ignition switch.