Even if you have inspected your brakes and you are sure they are working properly, there is another component to check before heading out to the water: lights. If your brake lights and turn signals are not functioning, then all the work you put into the brakes themselves will be for naught. If your brake lights are not working and you already checked the bulbs, it is time to check the wires. Because the wires are submerged so often, they can easily become corroded and may need to be replaced.
The first thing to do is find out what kind of wire harness you have. The most common configuration is called a “flat four” or “4 pole flat”. These harnesses have four wires: right turn signal/brake is green, left turn signal/brake is yellow, tail lights are brown, and the ground wire is white. Other configurations have five or more wires. The “flat five” contains a blue auxiliary wire often used for the reversing solenoid for trailer brakes. Once you have purchased your new harness and wires, remove the old harness, paying attention to how the wires were secured. Run the new wires inside the framing using the old wires to feed them through. The framing may have rings or tubing to support the wires. Yellow and brown wires run along the left side, green and brown on the right side. The white wire connects to any bolt behind the coupler and if you have a blue wire it connects to the solenoid.
Next connect the wires to their corresponding light. Taillights have a running light circuit (black or brown wire) and a brake/turn signal circuit (red or green wire for the right and red or yellow wire for the left). Connect brown to brown/black, yellow to red/yellow, and green to red/green. If you are planning to go into salt water, you will want to use a crimp connector and heat-shrink tubing to make these connections. Once all the connections are made, hook up the trailer and plug in the lights to make sure they work.
Some trailers do not come with a way to secure the wires within the frame. If you have one of these trailers, you will want to purchase galvanized wire clips. Make sure the harness is tight but not pinched anywhere. Give yourself enough slack that the plug will not pull out when you go around turns. Securing the wires to the frame will help to prevent chafing of the insulation and the wires from blowing in the wind.