Additional Notes
What's so special about this circuit? Well, the first third-brake light I installed I had to pull a wire from the Third Brake Light all the way to the brake pedal switch and I thought it would be easier to pluck the signal of both brakelights via the trunk. Bench-test your circuit first and apply power to the circuit, with the brakelight connected, for at least a couple of minutes. If the two SCR's are getting hot (depending on the type of bulb in your 3rd brake light), mount coolribs on them. I used the NTE5483 which is a 8A, 200Vrm type.