A 12-volt circuit is usually connected to a battery or a transformer. Because the voltage is relatively low, it is safe to troubleshoot if there is a short circuit without taking any extreme precautions. A circuit has to be complete for any electrical device to operate, so if you find your device is not working, there may be a short circuit. The best method to detect a 12-volt short circuit is to use a multimeter.
Check the power source is on and the electrical device is switched to the "On" position, even if the device doesn't operate. You need electricity to flow through the wires to detect a short circuit.
Turn on the multimeter. Insert the jack plug on the end of the red wire that connects to the multimeter into the "DCV" socket. Insert the jack plug on the end of the black wire that connects to the meter into the "COM" socket.
Refine the voltage setting using the dial on the front of the meter. Set it to read between zero and 15 volts.
Place the sensor on the end of the red wire onto the "+" terminal on the power source. Place the sensor on the end of the black wire onto the "-" terminal on the power source.
Read the meter display. If it reads 12 volts, the power source is operating correctly. If there isn't a reading, the power source is dead. If the meter fluctuates, it is likely there is a short in the power source, so it needs further investigation.
Check that the two wires connected to the power source are secure and are not touching each other. If the wires are touching, it is creating a short circuit so you need to rewire the connections.
Check the wires that connect to the terminal of the electrical device. Confirm they are secure. If they are loose or touching each other, it means there is a short circuit, so you need to rewire the connections.