When I added amber LED strobes as turn signals to my bike last summer, I also added a red LED strip as a brake light. It's triggered by a micro lever switch attached to the rear brake cable. It runs off 1 9V battery. Here's a rather blurry image.
Here's a picture of the micro lever switch attached to the brake cable (you also see the 9V battery that powers the brake light (it's attached to the rear rack). The best thing about this set up is the wiring didn't have to be run very long since everything's at the rear of the bike.
