This process will work with most stock stereos, and all aftermarket stereos. It can be modified to work with all stock stereos, but you may need a few more parts. I will show you how to do this with an aftermarket head unit (stereo).
When you look at the pictures, realize that I am using an amplifier that is too small for the subwoofer. It is for demonstration purposes and it does not sound as good as it should. In other words, my car needs a bigger amplifier to support the subwoofer.
Step 1: Parts
-Subwoofer speaker (see next step)
-Amplifier (see next step)
-Wiring kit (or each of the following)
-10 Gauge or thicker, 20 foot insulated wire (for power)
-10 Gauge or thicker, 3 foot insulated wire (for ground)
-18 or 16 Gauge, 15 foot wire
-RCA cables, 15 feet or longer (you need 2 or one with both red and white)
-In-line fuse, 50 amps or higher
-Some speaker wire
-4 small wood screws
For the power and ground wires, you will need thicker wires depending on how powerful the amplifier is. REALLY powerful amps could need as much as a 0 gauge wire. But I doubt any of you guys will be using an amp that big.
Walmart sells amplifier kits (as do some other stores). The kits tell you how many watts they can handle.
Step 2: Choosing an amp and speaker
What you want is the RMS of both the subwoofer speaker and the amplifier to be as close as possible. The RMS is the amount of power a speaker can continuously have sent to it without going bad. The RMS is also the amount of power the amplifier can continuously put out without over heating.
When looking at speakers and amplifiers, do not look at the peak power. A speaker or amp can only be run on the peak power for about a minute before it goes bad or overheats. You want to be running your subwoofers on its RMS rating rather than the peak rating.
For best sound, keep the impedance (Ohms) the same too.
Lets take the Kenwood KFC-W3011 for example. Its ratings are:
-4 Ohm Impedance
A good amp for this speaker (assuming this is the only speaker attached to the amp) might be the Rockford P400-2. This amp has the following ratings when it is in "bridged mode".
-4 Ohm Impedance
Choosing a size for a speaker is also important. Smaller speakers like the 8 and 10 inch ones, are very quick to respond and punch better than bigger ones, but are not very loud. Bigger ones like 15+ inch ones, are very loud compared to smaller ones at the same wattage, but have slow response, and make the sound more mushy. The bigger ones also handle lower frequencies better too. 12 inch speakers are a good compromise for a basic system.
Make sure you buy a subwoofer box that has a hole the same size as your speaker.
Step 3: Run wires - The most annoying part
Start by finding an opening in the car's firewall. This is the metal wall under the hood of the car, closest to the windshield. The other side of the firewall should be the inside of the car. I chose a hole that was behind my glove box and was fairly easy to get to from the engine compartment.
Run the majority of the power cable through the hole in the firewall, making sure to leave enough wire to get to the battery.
Strip the insulation off of the wire at the end where the battery is. Wire the in-line fuse to this end (if it is not part of the wire already). You want the fuse as close to the battery as possible. Do not connect the wire to the battery yet. Make sure you tape the place where you wired the fuse to the wire, so you don't get a short.
Run the remaining wire under the cars carpeting or through a wire channel, if there is one. You want to get this wire to the trunk of the car.
While you have the carpeting loose, run the 16 - 18 gauge wire and the RCA cables from the trunk to as close to the back of the stereo head unit as possible, leave a little slack at both ends.
Step 4: Wiring the audio
After you have the stereo out, look at the back of it. There should be 2 RCA connections. Run the RCA cables through the back of the center console and plug them into the 2 connections on the back of the stereo.
If your stereo does not have these connections, you will have to splice the wires into the rear speaker wires. Better yet, go out and by a new stereo that has RCA. They aren't very expensive any more.
Leave the stereo out for the next step.
Step 5: Wiring the remote
If you look at all the wires coming out of the back of the head unit, there should be 1 or 2 blue ones. These are called the remote wires. If your wires are labeled they may be labeled as:
or something similar to one of those.
If there are 2 wires, there should be one labeled Amp. If there is just one blue wire, you can use that. If you have a power antenna, you will have to splice into the blue wire for use with the amp too. What you need to do is connect the 16 - 18 gauge wire to the correct blue wire. When the stereo comes on, so will the amp.
If it is a stock stereo on a car without a power antenna and no blue wire, then run the 16 - 18 gauge wire to the fuse box, and connect it to a fuse that turns on when the accessories are on. Your amp will always be on when your car is on, but it shouldn't make noise, so its OK. If it does make noise (like from the engine), add a switch so you can turn it on or off.
Step 6: Put the speaker in the box
Put the speaker in the box, making sure the flimsy thin gasket thing is on it.
If the box has its own connectors on the outside, make sure they are wired to the speaker inside.
Screw the speaker into the box, using the holes on the outside rim of the speaker.
Set the subwoofer in the trunk of the car.
Step 7: Wiring up the amp
Connect the power cable from the battery into the spot on the amp that has one of the following markings (do not connect it to the speaker positive):
Connect the 16 to 18 gauge wire into the spot that says:
Connect the 3 foot, 10 gauge wire to the one marked (do not connect it to the speaker negative):
Connect the other end of the ground wire to a nearby bolt that connects to the body of the car.
Connect the speaker wire to the + and - marked for speakers on the amp. There may be 2 channels. If there are 2 channels and you can bridge your amp, do it. I won't explain bridging but its easy and you can Google it.
Connect the other end of the speaker wire to the + and - on the speaker box. Try to make sure that the + from the amp is connected to the + on the speaker, and the same with the -.
Step 8: Adding power
Make sure a large fuse is in the fuse holder.