Wire harnesses are used to connect two or more items, using a series of wires in one bundle. When one of these devices malfunctions, it's logical to test the wiring that connects to it. Using a multimeter is the best way to ensure that there is proper voltage and continuity along the individual wires in a harness.
Disconnect the end of the harness that feeds the device. For example, pull out the plug on the back of the vehicle's in-dash radio.
Set the switch on the multimeter to "DC."
Set the meter to the expected range of operation. In a mobile environment, that will be anywhere from 11 to 16 volts DC.
Touch the black (negative) lead to bare metal on the vehicle.
Touch the red (positive) lead to the wires in the harness that need to be tested.
Turn the vehicle to the "ACC" position with the key. Observe the reading on the meter. It should read within the 11-16 volt range; most commonly, 12-13.3.
Ensure the power to the harness feeding the device is disconnected.
Disconnect both ends of the harness slated for testing.
Turn the switch to the "resistance" setting on the multimeter. This setting is indicated by an image of an arrow pointing towards a positive ("+") symbol.