Do you have a testlight? If not, go buy one. Play around with it at the battery and see how it works. Then get under the dash and clip the one wire to a good ground, and then probe the two green wires going to the brake switch on the pedal arm. One wire should be hot all the time(light the testlight up all the time) and the other wire should light the testlight only when you push on the pedal with your hand.