I installed the "Brakelighter" third brake light on my 37 that has turn signals and brake lights on the same circuit. The light is LED, 12 inches long, split down the middle for left and right illumination. You connect three small gauge wires (in the trunk if you want). One for the left brake/turn, one for the right brake/turn, and one to ground. When you hit the brakes, both sides of the third light illuminate. When you active either turn signal, that side of the light will flash and the opposite side will illuminate steady. So, not only do you get a third brake, you also get left and right turn signals. It mounts on the inside of the back window, where it is much more visible up high. It is very bright. But, it does cost about 55 bucks. It comes in either 6 or 12 volt.
www.brakelighter.com