Rainman,
The Brake Sensor sits between the adjustor for the cable and the the actual brake caliper. It works like a car brake. Regardless of whether you have the unit turned on, when you press hit the brake, the light stays lit.

I'm not sure how you'd wire that into your 12V system but I do know that bike shops sell the brake sensor separately. i.e., you don't need a circuit board for the brake lite.