Wiring diagrams for a four wire trailer are available on websites such as How-To-Wire-It.com, AccessConnect.com and OffRoaders.com. These websites also provide information on how to wire trailers, trailer safety, hitches and 6 and 7 wire trailer diagrams.
In a four wire trailer system, the green wire connects the right stop and turn lights, and the yellow wire connects the left. The white wire is for the ground, and the brown is for the tail, running and license plate lights. Trailers with 6 and 7 wire systems include wiring for electric brakes, a 12 volt battery, and additional auxiliary power.
Be sure to route all wires so that they are tucked out of the way and can't catch or rub on anything. If the trailer has square tubing, running the wires through the tubing helps protect them and increases visual appeal. Some trailers come with pre-drilled holes, otherwise the holes need to be manually drilled in. In this case, make sure the holes are sealed when done to ensure that water doesn't fill the frame.
Stores sell specially made clips for U-channel and angle iron construction trailers. These clips attach to the frame and help keep the wiring out of the way.