I see by your profile you have a chev. if it came with the towing package it will already have the wire in place for a fifth wheel, at least mine did. Mine was bundled just about a foot in front of the driver side rear tire. I just drilled the proper sized hole about a foot infront of the rear tire in the inter bed wall. And connected the wires, they are paralled with the rear trailer connection at the factory. Easy installation.
Unlike most of the people who responded to your question, I put my connector just ahead of the wheel well on the driver's side (i.e. in between the back of the cab and the wheel well). My thinking was that I didn't want to be reaching between the rails of the truck and the bottom of the fifth wheel overhang to hook up my cable (could be very dangerous if you don't have a good hook up and things go bad). It works well for me and I would recommend it. I plug it in at the same time (and standing in the same place) that I hook up my emergency brake cable and put the padlock on the hitch after hooking up.
2006 Grand Surveyor 240BH
2004 GMC Sierra 2500HD 4X4 CC LB 6.0litre 4.10 gears
18K Reese/Hidden Hitch Signature Series Hitch
Prodigy Brake Controller
I installed mine where BeeTee is thinking. There is ample room for the socket, no structure damage, there is space behind the taillite for routing wiring and a clear path for wiring to the bumper hitch harness without drilling anything more than the socket hole. Works great, looks clean and is easy to reach. Use one of the intercepting plug kits available so you have both plugs intact. Measure carefully so the tailgate can close without striking the cable plug.