Fairly familiar with the wiring harnesses in that area since I added a backup camera to mine.
Both the left and right sides do have wiring harnesses from the front of the car, but none of the wires are particularly large and they are shared with other items, so I would avoid a high amp draw item like that in the back of the car.
I ran cabling along the ceiling (under the headliner, by the door frame) and it was quite easy. I would run dedicated wires for this project. I would go all the way across the ceiling, down the front a-pillar to the fuse panel that's to the left of the driver's foot. Then you could install a permanent 12V outlet, potentially with a high amperage rating. You could do a really slick install with a permanent 12V receptacle in the hatch area or you could install a high wattage inverter and a 110V outlet.