Expert Tips for Properly Wiring a Car Audio System

Aug 9, 2011

Knowing the right way to wire a car audio system is essential for delivering a quality system to your customer. This article will discuss proper phasing of the speakers to achieve the maximum bass response out of your speakers, as well as proper power and ground procedures to allow your vehicle to achieve the maximum voltage output to your amplifier.

Battery Power

Let’s start with the battery and the proper connections required to get the best performance out of a car stereo. If you look at the charging lead for the battery you’ll notice that it’s a smaller gauge wire than you’re using for the amplifier. It’s a good idea to upgrade it to a four-gauge power lead from the alternator to the battery so as not to restrict the potential power from the alternator. It’s also recommended that you look at the negative lead from the battery and, if a four-gauge ground lead to the chassis isn’t present, create a four-gauge ground for it as well. The grounding point for the battery should be a chassis ground with paint and dirt removed, and be directly to bare metal. This will allow the alternator of the vehicle to charge the battery to its full potential. It will also limit the voltage loss at the amplifier. Make sure all connections are tight and secure.

Fuse Selection & Location

With the battery and connections taken care of, let’s move on to the subject of the fuse for the amplifier. It should be located within 18 inches of the battery and is intended to protect the car in case the wire shorts somewhere in the vehicle. If the fuse isn’t present and a short occurs, the power wire has the potential to start a fire in the vehicle. The amperage of the fuse should be equivalent to, or as close as possible to, the amplifier’s current draw or onboard fuse. If you have an amp that draws 75 amps, an 80-amp fuse will work fine. Once again, make sure that all connections are tight. Remove the fuse until the install is completely finished. The last step of your install should be to place the fuse into the fuse holder.

Running Wire Through the Firewall

Now it’s time to locate a place to run the wire through the firewall of the vehicle. A lot of vehicles have unused factory grommets, which is the ideal location to run the power wire through the firewall. First, make sure that the open area in the center of the rubber grommet has enough space for the power wire to pass through. Next, remove the grommet from the firewall and cut an opening large enough for the power wire to pass through. Then reinstall the factory grommet and run your wire through the opening you created into the car. If an unused factory grommet doesn’t exist, you have to locate an area free of obstruction on both sides of the firewall and drill a hole. The gauge of the power wire will determine the diameter of the hole. Any time that you pass a wire through metal, there should be a properly sized protective grommet in place. This will prevent the sharp edges of the metal from wearing through the protective coating of the wire and causing a short in the wire.

Avoiding Wire Interference

Once you’ve found a way to get the power wire into the vehicle, it’s important that you take precautions to make sure the wire doesn’t interfere with any moving parts inside the car, such as the steering wheel linkage, brake pedal, gas pedal or clutch pedal. It’s also a good idea to double check from the inside of the vehicle to make sure that your grommet is in the proper location. Route the power wire toward the kick panel of the car. Remove the doorsill and kick panel from the vehicle and place both in a safe location where they won’t be stepped on or broken. Carefully start installing your power wire and the factory harness, if present. In some vehicles there’s a plastic channel for the factory wires to run through. This is a good place to run your power wire as it was designed by the factory to hold wires, so you know your power wire will be safe from any sharp edges that might lead to a short in the power wire. Make sure there’s enough space for your power wire to run along with the factory wires without creating any issues when you pull the carpet back over and replace the factory trim pieces.

Amplifier Location Considerations

Now it’s time to find a location for the amplifier. If you were planning on installing the amp under a seat in the car, it’s recommended that you look to make sure there’s enough room for the wires to get to the amp without interference from the stock parts of the car such as seat bolts or seat sliding tracks. If you were planning on putting the amp in the trunk of the car, make sure that the folding rear seats don’t have the potential to pinch the wires and cause a short in the system. Take the extra time to run the wires away from any moving parts of the car because it isn’t a lot of fun trying to trace a short that could be anywhere in the vehicle.

Delivering Signal to the Amplifier

Now, start on the signal to the amplifier from the source unit. Plan how many channels of audio signal you’d like to send to the amplifier. AAMP of America, for example, offers a wide variety of two-, four- and six-channel audio cables from Stinger and Sound Quest to help make your install as painless as possible. Once you’ve selected how many channels of audio you need to run, it’s time to prep the signal. You’ll need to include a remote turn-on lead with the signal cables to turn on the amplifier, which is done by running an 18-gauge primary wire along with the signal. If you pull a length of primary wire equal to the length of your signal cables, use electrical tape wrapped around both the primary wire and signal wires about every 2 feet. If using the stock speaker wires to the speakers to deliver, signal you can also include four speaker wires in the signal harness bundle. The four speaker wires can be used to do your rewire behind the radio and are already going to the stock speaker locations.  If you’re doing a high-power stereo setup, it’s recommended going directly to each speaker with upgraded wire to be better able to handle the power and not have the power restricted by inadequate wire. The 18-gauge primary wire will be hooked up to the amplifier-turn-on output from the source unit.

Reducing Noise

Once the signal is prepped, look at the audio signal cables that you’re using and check to see if they’re directional cables or not. If they’re directional cables, make sure that the arrows are pointing in the direction you’re sending the signal from the source unit to the amplifier. Remove the source unit and the factory panels on the opposite side of the vehicle from the power wire. The power wire and signal wires need to be kept apart; if they’re not it could lead to engine noise. Looking at the area where the source unit was located, check to see if there’s a clear path that the cables will run through without interfering with any of the stock components of the vehicle (air conditioner controls or linkage).  Now you can proceed to run the wires down the opposite side of the vehicle the same way you did with the power wire, again making sure not to interfere with the stock parts of the vehicle. It’s a good idea to avoid any factory electronic control modules because they can radiate noise that will be picked up by the RCA cables and create engine noise in the car stereo.

Testing Speaker Polarity

Once you have completed running the wires to your amplifier, you can go back to the source unit and complete the rewiring of the factory speaker wires if using the stock wires. If you’re not using the stock wires, you should run the wires directly to the speakers. To test which speaker wire goes to which speaker, you can use a 9-volt battery to “pop” the speaker (positive-speaker-wire-to-positive on the battery and negative-speaker-wire-to-negative on the battery). When doing this test you can also check the polarity of the speaker, if you can see the speaker. When you “pop” the speaker, if you’re using positive-to-positive and negative-to-negative, the speaker should “pop” out. The reason for checking the movement of the speaker is to make sure the speaker polarity is correct-you want all the speakers to move in the same direction to get the best bass response from your stereo. Once you have located which wire belongs to which speaker, you can hook it up to the proper channel of the amplifier.

Grounding the Amplifier

Now it’s time to find a good ground for the amplifier. The same rules apply as when the battery ground was addressed-you want a location that’s free of dirt, grime and paint. Before mounting the ground, it’s a good idea to look under the vehicle and make sure that you aren’t going to be screwing into any gas lines, fuel tanks or any other part of the car that doesn’t agree with a screw/bolt being driven into it. Once again, make sure that this is a good, tight connection. Route the ground wire to the amplifier and connect it. You can then hook up the 18-gauge turn-on wire that was run with the audio signal cables. Then, hook up your power wire and RCA cables, making sure that you have the RCA cables going to their proper location (front left/right, rear left/right and sub). Next, reinstall the source unit and place the fuse in the fuse holder. Now you can turn on the stereo and tune the system to the recommended levels. Reinstall your source unit and any factory panels that were removed to run the wires. and your sound upgrade is complete.

Rob Derry works at AAMP of America in Clearwater, Florida. For more information on AAMP of America and its mobile audio and video accessories, installation and fabrication supplies, go to