Brakes

I have a 1999 Chevy Suburban that I want to hook up a brake box for electric trailer brakes. I am not sure where to hook the wire up that is supposed to go to the truck brakes electrical system.

If you just need an electrical signal for "brakes on" then the brake light circuit will do it.

That is my problem, where is the brake light circuit? Anything at the pedal is not reachable.

For a trailer circuit, you're looking at the back end of the car. Tail lights are an excellent place to start.