How do you hook up trailer lights?

With some vehicles, you can purchase a WYE harness, which you install in the rear harness of the vehicle. You separate the provided plug and install the Y between the two plugs and the end of the Y becomes the trailer harness.

Other vehicles you have to splice the wires. Normally brown wire is the running lights, Yellow is the left turn, and Green is the right turn. White is the ground between the trailer and the vehicle, for a total of 4 wires.