It is plugged into either the bracket or the side of the brake light switch. Car is a 70 Challenger. I cannot tell as I had rotator cuff surgery and cannot get all the way down there to see it. It is the dark red terminal coming off on the side of the bracket.
The brake switch is a power source that is many times used for alarms or aftermarket audio equipment. I'll bet it had something to do with one of those things.
I know this wire must be serving as a ground of some sort as I have blinkers, tail lights with headlights on but no brake lights. If I tap into the brown wire running to the back the brake lights will work on one side.