Turn the key off to do electrical work like this. 12 volts isn't enough to hurt you (you can't even feel it), but you could still short out something by mistake. Many recommend disconnecting the battery.
For the tail lights, you need a constant power source. The license plate light is a good choice for this. In this step, I turn the clamp on the 3.5mm triple connector into a bullet so it can plug into the license plate power wire.
- Use your pliers to close the open prongs until they are small enough to plug into the 3.5 mm open end. Make it as round as you can for a good connection.
- Test the fit with your other triple connector.