Automobiles contain many different types of circuits. Each circuit uses electrical wires to carry both power and signal information to other components. Over time, these wires can become damaged due to misuse, accidents and component failure. These failures can cause both short circuits and open circuits---both of which render the wires unusable. The hardest part of fixing a wire involves locating the defective one---a process with which a multimeter can help. Multimeters are designed to test different parts of a circuit to locate the problem.
Disconnect the wires at their corresponding components at both ends using a screwdriver. Connect a jumper wire between one wire end and the car frame. Connect the red positive probe of the multimeter to the other end of the same wire; connect the multimeter's black probe to the car frame.
Set the multimeter to measure resistance; it should read zero resistance. If the meter reads "Open Circuit" or some other very high reading, the wire is bad and requires replacement.
Remove the jumper wire, and retest the wire. With the jumper wire removed, no part of the wire should contact the car frame. In this case, the meter should read "Open Circuit." If not, a short has occurred in the wire, and it requires replacement. Repeat this process for each wire.
Reconnect all of the wires except for the wires connecting to the end equipment. Set the multimeter to read voltage. Connect the positive wire to the positive red probe, and connect the black probe to a metal part of the car frame. Write down the voltage reading.
Consult an auto manual for the correct voltage needed at the device to which you originally connected the wires. Compare the measured reading with the ideal voltage as prescribed by the manual. If they are the same or close, the wire is fine. Otherwise, you must replace the wire.