The most cost effective first step is to buy an amplifier wiring kit off of an internet auction site, etc. This will come with a huge power wire, a short ground wire, a remote wire, and many times an inline fuse and various connectors to help keep the install organized and looking clean. Some stereo shops have huge spools of wire available by the foot. This is another cost effective alternative to buying a wiring kit, if you know the dimensions of your car.
2
Run the 12v power wire (usually the longest wire in the kit, mostly red, typically ranges from 8 gauge to 0 gauge) from the battery through the firewall and then to the amp. You can find a hole somewhere in the bottom right of the firewall. Don't connect the power wire to the battery or the amp just yet.
3
Find a solid metal ground somewhere near the amplifier. You'll need to stay within 2-3 feet of the amplifier to maintain the best ground connection. A way you can do this is pull up the carpet and scrape the metal until it is bare metal with no paint left on it. If the amplifier is to be mounted in the trunk, many times bolts from the suspension can be found directly above one of the rear wheels. These suspension components usually bolt directly to the frame, making them an ideal grounding point.
4
Pull the aftermarket CD player out of the dash. There is a blue wire with a white stripe hanging out of the back of the deck known as the remote wire. The remote wire is a simple 12v signal that the cd player outputs to tell the amplifier to turn on.
5
Take your remote wire that came in your amp kit and splice/solder it onto that blue and white-striped wire and run it through the dash and then along the door jam.
6
While you have the CD player removed, plug the RCA white and red plugs into the back of the deck where it says "Subwoofer Output." If your cd player does not have a "Subwoofer Output", or if you are using a stock cd player, you will need a device called an inline converter. It's a small box that has 4 input wires and the 2 necessary RCA output that will run to the amplifier. It takes the high level speaker voltage and drops it to a low level signal the amplifier can process. The 4 input wires can be connected to the rear speakers (+ and - for left and right).
7
Run all the wires straight back to the amplifier.you should run the power and remote control down the right as factory speaker wires run down the left due to fire risks if the power cable shorts out and you have your speaker cables down the same side it will blow your head deck (cd player). RCA cables should be run down the center of the car as RCA cables can pick up sounds from wire looms and also air con vents power wires.
8
Use Speaker Wire to connect subwoofers to amplifier. Gauge isn't extremely important here, as long as the wire is copper, the resistance-per-foot is in the milliohm range, meaning there will be miniscule, if any, voltage drop across the wire.
9
Hopefully by now you have a subwoofer box/enclosure. There are many different types of enclosures (sealed, vented, bandpass, infinite baffle, etc.). There are numerous articles explaining the pros and cons of each type of enclosure, far too many to list in this article. If you really want the best response possible, your subwoofer manual will list the ideal enclosure volume for each type of enclosure. If you don't want to go through all of the volume calculations, just buy a box slightly bigger, and stuff it with pillow stuffing from walmart until the subs respond the way you want them to.
10
Be sure to know the impedance of the subwoofer(s) you are using and try to match the amplifier's impedance accordingly. For example, if you have an amp that is 500w @ 4 ohms,