I've done this but it was an additional 4 Way onto the caravan towing electrics.
I have no idea if there is a suitable 12V power line going to the rear of the car. The problem with tapping into/onto existing 12V lines (switch or un-switched) is that it is easy/possible to overload that particular wire unless you know that a) that wire has the correct rating and b) that the wire that this wire may spur from has the correct rating.
You have three possible ways:
1) Run a wire from the from the from power socket. This may require tapping into the existing wire +12V wire. In the boot use a connection to chassis for the ground. No point in running two wires unless you want to.
2) Use a piggyback fuse takeoff connector in the fuse box for the fuse that powers the front power socket.
3) Run a fused wire from the battery terminal to the rear. This wire can also have a smart relay to detect when the engine is running. Uses battery voltage level to enable/disable the power feed.
