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Each trailer has at least four wires associated with the electrical system, and each wire color represents the light that each wire must connect to. In a four-way system, yellow wire represents the left turn lamp. This wire is also active when the vehicle brakes are applied. Green wire represents the right turn lamp and is active when braking. Brown wire represents the side marker, license plate and tail lamps and is active only when the vehicle headlamps are switched on. The white wire is connected to the trailer to serve as a ground wire. This is because most vehicles are made of aluminum or steel. Since neither metal is as good an electrical conductor as copper, the electrical ground is provided to the trailer via the white wire. A five-way system is nearly identical to a four-way system, but there is an additional (blue) wire. This wire can be used for auxiliary power (12 volts), or it may serve as an actuator signal wire for installed electric trailer brakes. A six-way system adds a black wire, and the wire color assignments differ greatly from the four- and five-way systems. The wire color assignments are as follows: Black: Auxiliary (12 volt) power, dome lights, battery charging terminal Green: Tail lamps, side markers and license plate lamps Brown: Right turn signal and stop lamp Red: Left turn signal and stop lamp Blue: Electric trailer brake actuator White: Ground A seven-way system is similar to the six-way system, but a yellow wire is added as an auxiliary power source. The black wire serves only as a dome light and battery charging terminal interface. Considerations When troubleshooting a problem with trailer tail lighting, several things must be considered: The trailer plug interface must be plugged in firmly to the vehicle plug interface; otherwise, the electrical circuit may not be complete, causing the trailer lights to malfunction. If any of the external trailer wires are visibly worn or broken, that wire will not carry any electrical current or will cause a short circuit to ground. Are the bulbs or LEDs damaged or burned out? These can be easily checked and replaced if necessary. If a light fixture has been damaged, the electrical circuit may be broken. Check the light fixtures for damage. The white ground wire must be connected to the wire harness on both ends of the trailer. The white ground wire should be connected to the trailer frame. If it is not, improper grounding may cause the lights to function improperly, if at all. Do the wire colors conform to the type of system the trailer is wired for? Ensure that each wire color represents the proper electrical connection.