You should bench-test a car stereo before it is installed, especially if it is a used unit. This will save you from trying to troubleshoot it after it is installed in the cramped quarter of the vehicle's dash. Bench-testing a car stereo is a straightforward process as speaker wiring is paired into colours that are standard across the industry. You can quickly bench-test a car stereo with a few test leads, a spare speaker and a power source.
Cut four, 2-foot jumper wires from the roll of electrical wire. Strip a half-inch of insulation from the ends of each wire with the electrical pliers.
Set the stereo down, with the display facing you. Locate the four pairs of speaker wires: two white, left-front; two grey, right-front; two green, left-rear; two violet, right-rear. The solid-coloured wires are positive; striped are negative.
Attach the end of one jumper wire to the positive wire of the test speaker with an alligator clip. Connect the other end of the jumper to the positive wire of the grey pair. Repeat with second jumper wire for the negative speaker wire.
Insert the antenna lead into the antenna socket on the stereo.
Attach one end of a third wire to the positive terminal of the battery with an alligator clip. Clip the other end of the wire to the red, yellow and orange wires coming from the stereo. Attach the fourth jumper wire to the stereo's back grounding wire, and the other end to the battery's negative terminal.
Turn on the stereo. Check the display for power. Adjust the volume control and left-right and front-rear fade controls while listening for the volume to fade in and out accordingly.
Turn the stereo off and move the jumper wires to the next pair or wires. Repeat for each pair of coloured wires.
If you have access to four test speakers you can connect them all at the same time by making more jumper wires.
Always turn the stereo off before disconnecting and connecting wires or it could cause a short-circuit in the speaker or stereo. Do not attempt to use a battery that is still connected to a vehicle, as a short-circuit can result.