We removed the lower dash panel (4 x 10mm bolts) to gain access to the brake light switch.
Under the hood, we drilled a small hole in the side of the fuse box to pass the 12 GA wire through (and sealed with silicone) the placed a 20 amp circuit breaker in a empty spot. The power feed was connected to the main power stud.
We used standard double jacketed high temp 12/2 brake wire in this install.
We used the black wire has 12 volt feed. This brake wire runs both through the firewall to the brake controller, and to the rear of the truck to the trailer plug.
We use a "side tap" to connect the 12 volt power from the circuit breaker to this black wire. This then provides a 12 volt power both to the controller under the dash, and a AUX 12 volt power feed to the plug at the back of the truck ("black" terminal)!
We then used the white wire as the "Blue" wire, as the brake controller output to the plug at the back (we just "pretend" the wire is blue, connecting it to the "blue" plug terminal and the blue wire out the back of the controller) as I do not have double jacked wire that is black and blue wire.
Remember, this is just a example of how "I" did it.
I dont claim it to be the right way, and your results may differ. it is just free information you to see and how you use it is up to you.