Customizing cars is a finicky thing. Sometimes you need to make things work when you only have limited resources. In this case, we're talking about wiring up brake lights from a single source, the third brake light. It's not too complicated to do, but it does take some time.
Go to the bulb mounted in the third brake light. There should be two wires going into the bulb. Have a friend hit the brake pedal and activate the light while you use a test light to probe the wires. When one of the wires lights up, you've discovered the positive wire of the circuit.
Now that you know which wire is the positive wire, you know by default that the other wire is a negative wire. Mark both wires for reference.
Cut the positive wire and strip back the ends to expose 1/8 to 1/4 inch of copper wire. Take a new wire and strip back the end as well.
Twist the new wire and one half of the old wire together and use the butt splice connector and crimping tool to join the two together. In the remaining hole, crimp in the other half of the original positive wire.
Repeat this process for the negative wire, using a separate color wire.
Take the two wires and run them towards the rear of the vehicle, securing the wire to the chassis with zip ties along the way. If you prefer a neater look to the wiring, before you crimp the wires to the third brake light wires, have a friend hold the two wires together. Go to the other end of the wires and hold them taut. Place your two wires into a drill and tighten the chuck. Spin the wires in the drill until they turn into a nice braid.
Once the wires are at the brake lights, wire the positive wire to the positive wire on the brake light, and the negative wire to the negative wire. Have a friend step on the brake pedal to confirm that you've wired everything correctly. If not, double check all of your connections and use a test light to troubleshoot.
