As far as I am aware there is no constant power feed (or even ACC feed) down the back end of the truck... Everything down the back end is lighting (which I would not want to tap into, depending on what you are doing with it)
Unless there is a permanent feed for your trailer plug, but I dont think this is included in the standard loom.
Yeah - nothing constant. The closest you'll get is ACC from the centre console cig socket. This is easy to get to - just lift out the centre coin pots, and then you'll see the wires to the cig socket. Tap into these, then it's easy to jam wire under the carpet, beneath the seat, along the rear passenger door sill and up out through the breather holes behind the back seats.