Cut the plug off and splice the gray and black/yellow wires together. Fit the gasket on to the light and have someone hold it to the body while you run up the 2 screws from underneath. Run the wire over and through one of the holes in the drivers side hinge, and then under the electrical panel.
The easiest place to access the brake light wiring is at the back left side of the engine compartment near the plug that goes out to the lower brake lights. On my car the brake lights are a black/yellow wire, you will want to check the color code for your car. Splice the grey wire from the 3 rdbrake light onto the wire for the existing lights. Also splice the brown to any other brown wire. All of the brown wires on the cars should be ground.
Test the lights, and enjoy the added brake light visibility (Figure 3).