Trailer Wiring + Brake Controller

I am trying to hook up trailer brakes on my '97 GMC 1/2 ton with the tow pkg.
After searching this forum for info, I have found my blue(trailer brakes) and orange(aux pwr) wires that run from the rear bumper to the engine bay.

Now, at the back bumper end, there is ALSO a white wire that is hot with the brake switch. I need a brake switch lead in the cab for the trailer brake controller. So why is it back here??? :confused:
Any idea what this is for - and if it might be found somewhere in the cab so I can use this lead for the controller???

Any suggestions appreciated....
Thanks in advance.

With standard trailer wiring, white is the ground wire, so obviously it should not be hot. Has the wiring been worked on before, and does it work for the lights when connected to a trailer? It's possible it could be for a high-mount brake light if a topper is installed :dunno:

For the brake input to the controller, I would just splice into the brake light switch wire at the brake pedal. Some vehicles even have a female spade-type connector spliced into the wire for this reason, so check before you go cutting and soldering.

No, it's for the brake lite for sure. It had a tag on it that said: WHT WIRE BRK LEAD. I was hopeing I could find it in the cab somewhere since it doesn't go to anything. Probably for a third lite on a shell. :( Where might this female spade be located in the cab? I prefer a nice clean install.