How to install a car subwoofer to eliminate problems.
1) Disconnect the negative battery terminal on your car. This should always be the first step in any installation. Period. This eliminates any potential electrical flow in the vehicle and will prevent the sundry computers and devices in your car, in addition to your stereo, from being damaged by your tomfoolery. If you're going to connect or disconnect anything from the car's electrical system you should either do this or pull the fuse between the battery and that component. When in doubt, pull the terminal.
2)With the woofer box in the car, place the amplifier. For easy installations I like to affix amplifiers to the back of folding rear seats. That way they're out of the way until you need to work on them, but when you do, you have access to the whole thing. Rear seats don't usually vibrate much either, which can be a serious danger to an amplifier. Mounting an amp to a woofer box is often a bad idea; the walls flex with music and cause the amplifier to vibrate. These vibrations can damage solder connections and eventually cause the amplifier to self-destruct.
More dangerous than vibration, however, is heat. An amplifier in tight confines cannot effectively dissipate heat. Most amplifiers (Class A/B) are extremely inefficient and produce fantastic amounts of heat. This is why most amp chassis are generous heat sinks. If there isn't enough air around the amp to absorb the excess heat, heat damage may occur. Placing amps that run hot underneath seats is a no-no and not even a particularly good way to jury-rig heated seats for the winter. Class D (so-called digital amplifiers) amps usually do not produce nearly as much heat and can be placed in tighter confines. If you put an amp in a tight spot, be sure to check it after playing it for a while to be sure it isn't getting too hot.
You should also avoid mounting the amp directly to the chassis of the car. This can cause ground problems, since the car uses the body as a ground and your amp often uses its chassis for the same purpose. If you have trouble, mount a piece of wood to the car, and then affix the amp to that. Instant amp-rack. Once mounted, connect the speaker wires from the woofer box to the amplifier. You may need to use terminals for this, but most amps let you use normal stripped wire.
3) Run the power cable. This is the fun part. By "fun" I mean this is often the part where you lose small pieces of your fingers, discover vile pockets of debris, and learn that there is a surprisingly large amount of crap hidden underneath car carpeting. Affix a ring terminal to the end of the cable at the battery. Connect this to the positive terminal on the battery. Now, within a foot of that connection, find a place to put your fuse holder. Make sure the hood (bonnet) won't hit it, but be sure you can get into it easily. You want to be able to replace it without trouble. Fuses are vital. If a wire breaks and comes in contact with metal, it will blow the fuse. If the fuse is not there, the wire will either catch on fire or melt due to the massive current flowing through it when it shorts, or the battery will boil and explode. These are bad things. Always use fuses.
Before poking the wire through, you'll need to pull up the carpet and remove some interior trim pieces. Most of the plastic interior parts of a car are held on with screws. You'll want to pull off the doorsills (the plastic at the bottom of the where the doors meet the body of the car) and probably the kick panels (the plastic plates that cover the sides of the interior of the car in the foot wells). With those out of the way, you can move the carpet at your leisure. Snake the wire under the carpet and to the doorsill. Most wire that is 4 gauge or smaller will fit under the sill and can be run alongside the edge of the passenger compartment all the way to the rear of the car.
Many guides advise that you not run signal wires alongside power wires. Good modern RCA cables reject most noise, and it is little known that since the entire body of a car constantly conducts electricity, it is actually emitting more interference than most any audio power cables. I usually run my power cable outside the car, since I use 0/1 cable (on other vehicles I go side-by-side). I won't cover that process here, but it is somewhat more involved than running cable inside. For most applications, running power and RCA side-by-side is fine. If you find there is noise in the system, you may consider other options.
Snaking wire under carpet is very easy. Just push and pull the wire from wherever you can reach it until it is near the end of the car. You can usually just run it under the back seats and have it come out in the trunk. Stretch it to the amplifier and check the distance. Add a couple feet for safety and then cut it. Nothing is worse than having to move your amp or box later and not having enough cable to fit that length. Depending on the amplifier, you will either connect the wire to it bare or with a crimp-on connector, like a spade. Connect the amp now. Don't worry, with the fuse out and battery disconnected, there is no power to the device.
Now we run the ground. This is very easy. Find a section of bare metal or a strong bolt that goes into the body of the car, within two or three feet of the amplifier. Shorter is better, but make sure the cable isn't taut. It's no fun to rip out your power terminals. If you go to bare metal, brush it so it is shiny and exposed, crimp a ring terminal to one end of the ground cable, and use a self-tapping screw to fix it to the body. If you use a bolt, polish the bolt and the hole, then use a ring terminal between the bolt and body. Connect the ground to the amp. Easy.
4) Run the Remote and Signal Wires. For this part, you'll want to pull the radio out of your dash. This may be rather involved. In the back, you'll find RCA outputs. Check the manual for your receiver to see which are the right ones to use. Remember this. Take the RCA cables and snake them either up into the dash from the foot well or down from the receiver, whichever looks easier. Connect them to the deck. At the same time, take your remote turn-on lead and connect it to the remote wire or antenna turn-on wire in the back of the receiver. Do not connect this to a constant power source, such as teh battery. Yes, the amp will work that way, but if the power is constant, the amplifier will always be turned on. This will quickly result in a dead car battery and a grumpy you. Use a butt connector to join the remote lead to the remote wire. If you don't know what these are, look them up; they're the most useful electrical connector known to man (I use hundreds every year).
Run the RCA cables and remote wire together, back to the amplifier. Connect them to the amp as described in the manual. Replace the receiver back in the dash of your car. Now double-check all your connections, move the wires exactly where you want them and where they will fit, then reassemble the interior of your car. The hard part is now done.
5)Test everything and choose settings. Reconnect the negative battery terminal. It may spark a little bit. This is normal. All the various components in your car are suddenly clamoring for juice. Don't put the fuse in the fuse holder yet. Go turn accessory power on in your car. Make sure your radio and accessories work. If anything doesn't, check for blown fuses in your vehicle's fuse box. Replace any blown fuses. If it blows when you put a new one in, you've got a short circuit somewhere. This means you've probably nicked a wire you weren't supposed to or connected something wrong. Recheck everything. If everything works fine, hooray, your car has possibly survived your tomfoolery.