Go to Walmart or any hardware store and buy a test light. Test light is a probe, there is a needle on one end, a light and a ground wire. Clamp the ground wire to any bare metal that attaches to the frame (axle nuts are good for this), stick the probe into one of the wires running off the bike, then press on the brake. If the test light lites up, you've found your brake wire. Go through all the wires until you get everything hooked up.