Installing the correct wiring from your tow vehicle to the trailer is of the utmost importance. If the lights on the trailer do not work, or work incorrectly, you could have a ticket on your hands at the least, and a disaster at the worst. On most vehicles it isn’t very difficult to wire in trailer lights, especially once you understand what the goal is.
Locate and identify the proper wires. This is usually best done by removing the tail light assembly and determining which wire set goes to the tail light, which wire set goes to the brake light, and which wire set goes to the blinker. Once these wires are identified, you can label them with masking tape and a marker so that there is no need to find them again.
Splice the trailer wiring into the existing wiring using connecting butts and a wire stripper / pincher. This can be done in either of two ways. You can cut the wire completely and then put a barrel type connecter from the vehicle wiring to the trailer wiring, or there is a connector which will clamp down over two wires and cut through the insulation, effectively connecting them with a solid metal piece inside the plastic unit. It is recommended to use the latter type, because they are quick, easy, and require less work to install. Once all the wire is spliced in, run it through the vehicle to the outside in such a fashion where it won't become pinched and ground, because a car fire could start. Always try and either tape up the wires good using a lot of electrical tape, or run the wires through a plastic loom.
Connect the wires to the trailer connector. Whether you choose to have a round pigtail type connector, a 4-in-1 style which is flatter, a seven-pin style -- whatever the choice, it has to be wired into the vehicle wiring, and it has to match the connector already on the trailer. In this instance, since there is one wire coming out of the vehicle, and one wire coming off the connector, it is a great idea to just use barrel style two-way electrical connectors for the splice. Make sure the connector does not hang down too low or get pinched, and make sure there is a safe place to store it when not in use!