Do you have the Big Tow package?
If you do, you can buy this adapter that turns the trailer wiring outlet into a 12v outlet. The opposite end of the plug (you can't see it in the picture) is where you plug the 12v accessory in.
If you use this, you won't have to do a lot of drilling, wiring, etc. I think this part is good because you can have easy access to the plug even if the bed of your truck is full of dirt, lumber, etc.
http://www.towingtown.com/catalog/p2219.html
