Trailer electrical systems have two parts: the lighting system, which all trailers have, and the electric braking system, which your trailer may or may not have. The power for the trailer's electrical system comes from the tow vehicle, through the connecting plug. Use a step-by-step approach to diagnosing trailer electrical systems, ruling out trouble with each component and proceeding until the faulty area is found.
Connect the ground terminal of the test light to a metal part of the tow vehicle that is connected to the frame. You can use the receiver, bumper or the frame itself.
Turn on the headlights of the tow vehicle. Use the probe end of the test light to touch the terminals of the trailer connector on the tow vehicle one at a time. When you touch at least one terminal, the test light should light. If it does not, the wiring in the tow vehicle has a fault, which must be corrected before going further.
Plug in the trailer electrical connector to the tow vehicle plug. With the headlight switch turned on, assess which lights are not working. Turn the key on and check the indicators to see if they are working. Have a helper push on the brake pedal to make sure that the brake lights are working. Make a note of any lights that are not working.
Remove the light bulb from the light that is not working. Inspect the bulb for a broken filament wire inside, as well as black surfaces on the glass. These may indicate a blown bulb. If possible, try a new bulb and see if it works.
Test the light socket for power using the test light touching the probe to the electrical contact in the bottom centre of the socket. If it does not light, trace the wire by hand toward the tow vehicle power connector, looking for breaks in the wire, or corrosion. Check all connectors: they should fit tightly and free of corrosion. Expose the wiring at each connector, and check for power with the test light. When the test shows electricity present, the fault exists between that point and the connection that you tested before.
Check the section of wire that you have established as having the fault carefully. If you still can not find a break in the wire, connect a jumper wire between the two points. If the light functions, replace that section of wire with a new wire, as the break could be hidden.
Troubleshoot trailer brakes in a similar manner. Start at the brake magnet, and work toward the connector. Be certain when diagnosing trailer brakes to have a helper slide the brake application lever on the controller to apply the brakes and energise the circuit.