An electrical short in a wiring harness can occur when the conductors of two or more wires make electrical contact. This is usually due to physical damage to the cable where the insulation has been cut or abraded away, exposing bare conductors. Finding a short can be accomplished by testing the continuity of the wires in groups, then continuously dividing the groups in half until you are left with only the wires containing the short.
Set the multimeter to continuity mode. On most multimeters, this mode will give an audible beep when the probe leads are touched together.
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Isolate the wires on one end of the harness so that no bare conductors are touching each other. If it is difficult to do this, you can try isolating the wires by taping them to a non-conductive surface, like a table top.
Group the wires on the other end of the wiring harness into two separate bundles.
Use alligator clips to electrically connect all of the bare conductors in each group.
Clip the black probe lead of the multimeter to one wire bundle and press the red probe lead onto the other wire bundle. If there is a short, you will hear a beep.
Separate one of the bundles into two smaller bundles. Keep the other connection intact. One of the newly formed smaller bundles will cause a beep, while the other one will not.
Separate the bundle that caused the beep into yet two more smaller bundles of conductors, testing each of these new bundles in the same manner. Proceed in this fashion until you are left with one wire.
Use the method employed in steps 6 and 7 to pare down the wires in the other large bundle. You will eventually be left with the two wires that are shorted together.