I just installed the Hopkins 43355 trailer wiring adapter in my '09, and there's a couple of pointers for anyone else doing this.
1. The instructions say you will find the wiring connections "...6 to 12 (inches) underneath the vehicle behind the rear bumper...". You won't find them there. The connections are actually mounted to the taillamp housings, which you will need to remove.
2. Removing the lamp housings: Open the tailgate and look for three black bolts with 10mm heads. The top one and bottom one are the lamp housing bolts. Remove them. You do not need to remove the center one. The housing will now be loose, but not free. You will need to carefully pull it straight back (don't pull at an angle or rotate the housing) and the two pop-in fasteners will pop free.
2. Snake your adapter wires up through the holes where the factory wiring goes. Route the wires along the existing harnesses underneath the truck wherever possible.
3. Apply a little electrical grease to the female connection points and the inside edge of the connector housings for the electrically male connectors.
4. When reinstalling the lamp housings, you should try to tuck the connectors into the open space as you are installing the housing. Align the push in connectors and push them in until they pop into place. Reinstall the two bolts hand-tight while holding the housing in place.
Note: The kit was probably designed for trucks where the connections were underneath the truck, with a pigtail leading to the lamp housings. Since the '09 (maybe earlier) has the connections in the housings, there is little to no "extra" wire, so you will need to be careful how you run the wires, especially the passenger side. I considered soldering in an extension on the passenger side, since it is a single wire leading to the adapter, but I made it work as is.