Toyota Tundra Brake Light Wiring

Paulster is right, I'm just going to expand on it a bit. The light on your cap will only light up when the brake is pressed, that's why there's only two wires. One of the two wires is a ground (likely black). Connect this to the ground for your tail lights (also likely the black wire).

Then to address the two green wires, find out which wire gets power only when the brake peddle is pressed. Whichever that wire is, that is the one you want to hook up to your cap. The good news is, if you mix up the green wires it's very unlikely to hurt anything. If the cap light always stays lit, you got the wrong one. If it never lights up, the ground is wrong.