I just installed a Tekonsha P3 brake control in our coach last week. It did not have a trailer brake prewire. I had to find a source for the brake light input (used the air pressure switch) , a 20amp 12v source, a ground, and the big one was routing a wire from the drivers area to the trailer light connector near the trailer hitch.
My grandson and I spent a lot of time routing the trailer brake wire. On our coach, the wiring is routed up between the frame rails. We used a Garner Bender electricians fish tape tool to pull the wires through.
I used low resistance 12 gauge aircraft wire from Wicks aircraft. I ordered 60' and used just over 50'.
Related issue: We also installed a new Hopkins 48470 trailer light connector. Glenn is correct about the trailer brake lights. My coach has separate brake lights from the turn signals. I need to go back and install a 3 to 2 wire converter for the trailer brake lights to work properly.
Look at post 19
Hope this helps.