Check etrailer.com. They specialize in RV related parts and towing. They have lots of ready-made harness to plug into the factory harness without cutting. I had my Edge set up to tow behind my motor home. They had everything we needed and I don't think we cut a single wire on the car.

 

There should be extra wires near the spare tire for just this purpose. I believe the ARE website even has a write up of using them. That's where mine is connected to.

After a quick look at the ARE website, they've apparently taken that info off it. Any dealer should be able to give you the specific info. But you should find a bundle of wires there above or near the spare tire that has a brake signal wire in it. If you got one with the power locking rear glass, the signal for it comes from the passenger side front door sill plate or kick panel area.

 

Don't splice in to the truck tail light wiring or you will get a blinking brake light when you use the turn signal.
There is allegedly a blue-orange wire under the hood near the hood open switch on the driver side for a brake signal/power connection.

The older trucks had a green wire poked out of the tail light harness with a label CHMSL for center high mount stop light.
If you are unable to find either of these, the truck should already have a trailer harness back there so you could splice off the trailer brake wire. This may also be preferable to running another wire to the back of the truck.

 

I had a used cap on my truck and just wired it into the tail light and I had a blinking brake light when the turn signal was on. When my truck was keyed I replaced the cap and the new one seems to have a control box mounted to the rear window frame to stop that from happening.