Electrical wiring is an essential part of every trailer because a basic, light system is required by law. Every road-bearing trailer must have marker, signal and brake lights. Because these systems are directly exposed to the elements, they have a tendency to fail when you need them. Testing the electrical system on your trailer before every trip is a good habit to get into. Once you discover a problem, finding the short may require some troubleshooting.
Connect the trailer's electrical system to a dependable vehicle's electrical connection.
Look at the trailer's ground connection. If every light doesn't work or they work dimly, then the problem is most likely a faulty ground. A strong ground connection must be made to bare metal.
Check the light bulb in the affected system if the problem is isolated to one light. A good bulb will have a thin wire connecting its contact wires inside the bulb. If this bulb is burnt out, replace it.
Follow the electrical wire from the light to the first connection point. Check if there is power before the connection with the 12-volt test light. If there is power, rewire this connection. The 12-volt test light works by connecting its ground to the trailer's frame and using the probe on bare wire to search for power. When the test light illuminates, it has found a power source.
Continue following the wire, checking all of its connections in the same way as Step 4. While you are following the wire, look closely at its insulated cover for any damage that exposes the wire. Make repairs as necessary.