adding trailer brakes

I have a 97 sierra 1500 z71. I found a white wire at the back of the truck that works with the brake lights. It is a lighter guage wire, but could I use that for the brake wire on the the trailer brake controller intead of splicing into the brake pedal switch?

I would say if the controller's instructions say the power should come off the switch itself, then that is what you should do, instead of guessing about the white wire and possibly creating a situation where you would lose trailer brakes.

The electric brake controller should be mounted on or under your dash, so you have access to the adjustment switch. Unless I misunderstand you, isn't it easier to run a short wire off the brake switch instead of running a long wire back from the rear of the truck?

Just thought I could use that since the brake pedal switch is hard to get to and might be better than trying to splice into another wire. Thought maybe I could find the wire in the front and just run it into the cab.