Feed the wire through the spoiler so that it comes out of the bigger hole on the bottom (it's on the left side), and connect it to a harness that you will find in the lining of the trunk. First, disconnect your other 3rd brake light (the one on the car) by lifting up your hatch, and finding a rubber grommet. It's circular, and you just pull it out, and pull out some wires, and disconnect the clip that seems to go to the brake light.
Also, if you connect the spoiler light to the harness that is already present in your trunk (you can find it with the wiring near your License Plate
light wires on the inside of the hatch), and the light on the spoiler isn't lighting up, then you will have to do what I did.
You will have to feed the cord through the left side of the hatch, and connect it to the wire that the old 3rd brake light was connected to. This was a pain in the ass, so here's how I did it.
I fed some weed wacker wire through it, then once it was through, tied the wire to one end, and carefully pulled it through. This took a while since that side of the hatch is already packed with wires. Just take your time, and hopefully that the harness that is already hooked up on your car works, because mine didn't, and that's why I had to feed it through the hatch.