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.