Wiring your car stereo system yourself is a relatively simple process for those with even the most basic knowledge of electrical wiring or automotive repair, and is a great way to save money on your car stereo system.
Understand the basics of all car stereo systems. A car stereo system consists of 4 main components and the wiring that connects them. These are the head unit, the main speakers, optional amplifiers and the subwoofers, which are also optional but generally considered a necessary part of any good system.
Know that the core of any car stereo is the head unit, which is the cassette or CD player that goes in the dash. All the other components are connected to the head unit by at least one pair of wires.
Know about car speakers. The main speakers are usually 2 in the front and 2 in the back, although with what are known as component speakers, each speaker is broken down into two speakers: one for the highs and one for the bass. Each speaker or speaker set (known as a channel) connects to the head unit via a pair of wires. Generally if you are installing a new head unit you want to install new speakers.
Know about amplifiers. Amplifiers provide extra power to your speakers and/or extra channels of power for additional speakers. The most common use for an amplifier (amp) is to power subwoofers. The amp connects to your head unit via an RCA cable and often a "remote on" wire, and also connects directly to your car battery through a fused "hot" wire and to the car's chassis with a short ground wire. This article assumes the use of a single amplifier connected to a pair of subwoofers. If you do not have these components simply ignore the steps specific to the amp, doing so will not affect the rest of your installation.
Know about subwoofers. Subwoofers are part of any good stereo system. They provide the deep bass that small speakers cannot achieve. Subwoofers are connected to the amplifier which can usually be mounted right to the subwoofer box. If you do not have subwoofers you can simply ignore the steps specific to them.
Be properly prepared. This is extremely important since you do not want to realize you're missing something in the middle of the installation.
Purchase the following: A car stereo wire harness which connects the existing wiring for your old head unit to your new one (this will be specific to BOTH the model and year of your car AND to the make of your new head unit); an amplifier wiring kit which will contain all the wires to get power and signal to your amplifier; six feet of 14-gauge speaker wire for the subwoofers. Your speakers should have come with their own speaker wires.
Obtain a guide to removing the dash, doors panels (or whatever compartment the speakers are in), and floor molding of your car. You may be able to find a guide for this online. If not, you can get a repair manual for your car at most automotive stores and online).
Gather the following tools: Screwdrivers to fit the job, pliers, wire cutters, wire strippers, a utility knife, sand paper or a file, electrical tape, a 9-volt battery, and any other tools that the instructions for installing the head unit and your car guides specify.
Disconnect the ground (negative) cable from your battery first and foremost. NEVER work on the electrical system of your car with this connected.
Install your new speakers. Start by removing the paneling of the speaker enclosure. Unscrew and disconnect the old speaker.
Ideally the speaker wire will be connected to the old speakers with a metal clip that can then slide right on to your new ones. If not, cut the wire off the old speakers, strip about half an inch of it, then cut the last 6 inches or so off the speaker wire that came with your speakers so you have the proper connectors with a little wire attached, strip a half inch of those wires and twist connect them to the existing speaker wire. Bend the twists so they are in-line with the wire and wrap each connection in electrical tape so no wire is showing to protect them.
Attach the connectors to your new speaker. Make sure to connect the negative wire (-/black) to the negative terminal on the speaker and the positive wire (+/red or white) to the positive one. Screw the speaker in place. If you have component speakers, and therefore a crossover, make sure the crossover is secured in the speaker compartment so that it does not bounce around while you drive.
Wait to put the speaker enclosures back together until you have installed the head unit.
Follow the instructions you got for removing whatever part of the dash covers it and take out your existing head unit. Connect the car stereo wire harness to the connector that was attached to your old radio and attach the other end to your new one. Connect the antenna cable (the single wire with the big plug at the end) to your new head unit as well.
If the speaker wires are separate from the harness connect them using the method of stripping, twisting, and taping described above. Do this one at a time or use masking tape to label the wire to avoid confusion. If you are having trouble figuring out which wires go to which speaker or which is positive and negative read the tip at the bottom of this section.
Do not put your new head unit into the dash yet-you still need to connect the amplifier to it. Put the speaker enclosures back together now.
Attach the thick positive (+/red) power cable for your amplifier to the connector on positive terminal of your battery. The kit you purchased should have come with an o-ring on one end of the wire (you may have to crimp it on yourself, if so do it on the end with the fuse), this ring can be sandwiched in the bolt on the connector that connects your car to the battery.
Put the fuse in the fuse holder.
Run the power cable through the firewall of your car (there is usually an opening on the drivers side) and along to the back of the car where your amp is. It is usually best to run this wire under the plastic molding that goes along the bottom edge of your car, you never want the power cable to come near any speaker wires. At the same time run the remote on wire (a really thin wire from the amplifier wiring kit), through the dash where the head unit will sit, along with the power cable to the amp.
On the other side of the car run the RCA signal cable through the dash from the head unit to the amp (it's a pair of wires together with connectors on each end).
In the back of the car pick exactly where your amp and subwoofers are going to go. Part of choosing this is that the thick black ground cable (negative) needs to have a place to connect to the metal frame of your car. This should be as short as possible, you never want the ground cable to be longer than 3 feet.
Find a good screw or bolt, take if off and sandpaper the metal surface that the o-ring will make contact with, then screw it down tightly.
Attach the RCA signal cable and the remote on cable to your head unit and attach the power, ground, RCA signal, and remote on wires to your amp.
If you have not already done so, place your subwoofer box in the trunk and attach your amplifier to it if that is what you are doing. If you are not going to screw your amp to the subwoofer box then it must be secured to something else.
Attach the 14-gauge speaker wires to the left and right channels of the amplifier and to the speaker connectors of the subwoofer box.
Mount your head unit into the dash. Double check all of the connections on the back of it to make sure they are secure. Then slide your head unit into its slot in the dash and screw it securely in place. Before you re-attach the dash, reconnect the ground (negative) cable to your car battery and turn the stereo on and move around the car listening to each speaker to make sure everything is working properly. Then put the dash back on. You are all done!
Guides to purchasing all of the above mentioned components can be found right here on eHow. Always read the installation guides that came with your new car stereo equipment, especially the one for the head unit. They will give you additional insight into the proper way of how to wire your car stereo system. If you are preparing to hook up speaker wires and you are not sure which is negative and which is positive or which wires go to which speaker, take a 9-volt battery and touch the speaker wires to the the battery. One of the speakers will move. If the speaker moves forward (or out), then you have touched the positive lead of the speaker to the positive terminal of the battery. If the speaker moves back (or in) then the wire touching the positive terminal of the battery is the negative speaker wire.
Never have your speaker or signal wires run near your amplifier's power cables. If they must cross do so perpendicularly and only in one spot. Never EVER work on the electrical system of your car with the ground wire attached to the battery. You can damage yourself and your equipment.