I was a lil pissed when my 07 2500 sub didn't have it built in (meanwhile my buddy's 07 2500 sierra did...) there's 4 wires under teh dash taped up together red blue white and black, you wire those to an aftermarket trailer brake controller, and then you get to connect the power onto the terminal under the hood (think its orange and all taped up). Did that the second night I had the truck home from the dealer.
Then I proceeded to throw a 10K trailer on it with about 100 miles on it. Luckily the new trucks (if you have the 7 pin plug there already) have seperate fuses for the trailer wiring, so if there's a short on your trailer it doesn't take out running lights on the truck or signals on the truck etc.
the previous body style had a connector which you had to buy teh extra half of, but the 07+ just have 4 wires taped up, 4 butt connectors and you're done though.