My 2009 2500 HD has the wiring harness under the bed just behind the cab on drivers side. It is part of the tow package.
The moron that installed my 5th wheel hitch put the plug with a splice in the back near tailgate. Thank god he used the plug style to splice into the rear light plugs and didn't cut the wires. Why in the heck didn't he just use the harness to a 7 pin plug?
If I knew he was going to do it that way I would have saved him the effort (and me $$) and done it myself. Some of the installers aren't the sharpest tacks in the box. Avoid Truck Trailer and Hitch in KC.