I'm going to show you how to run a car stereo using an old PC power supply. If you're bored and want a project, or you're like me and just like making things instead of going out and buying purpose built ones, this is for you.
If you manage to screw up your stereo, speakers, power supply or anything else, don't blame me.
Also, this is my first Instructable, so it was kind of a learning curve on how this website works.

Step 1: What You Need

You'll need:
Car stereo- I'm using a Sony Xplod CDX-R3000
Speakers- I have a set of Acoustic Research and a set of RCA speakers
PSU- old computer power supply
Wire strippers
Wires
Speaker wire
Electrical tape

Step 2: Wire Your PSU

To start, find the large connector on your PSU.
Now, find the green and black wires right next to each other. Put a piece of wire between these as a jumper. This will make the PSU power up when you turn the switch on.

Step 3: Wiring Your Stereo

To begin with, get the red, yellow and black on the stereo
Red is ignition, yellow is constant and black is ground.
Twist the red and yellow wires together. This is to make the stereo power up.

Step 4: Stereo and PSU Meet at Last

This is where the stereo and power supply meet at last
Find one of the four pin connectors, with the yellow, black and red wires on it
You need a yellow and black wire next to each other
The black ground on your stereo goes to the black wire on the connector
The red and yellow on the stereo go to the yellow wire on the connector

Step 5: Wire Speakers

This is about as simple as it gets. Almost all car stereos follow the same wiring diagram. Just hook your speakers up the same way.
You can solder all the wires for a permanent installation, but I'm constantly changing what I have so I just twist and tape.
You won't need the blue antenna wire, brown (if yours has one for muting), blue with white line unless you add an amp and sub, light green (parking brake) if you have one, or orange (illumination) as most aftermarket stereos don't have an illumination wire.
Gray and white are front speakers, green and purple are rear speakers. Negative has a black strip on it, positive is a solid color.

Step 6: Plug It In, Power It Up

The time has come to test it.
Plug the cord in, and hit the switch, and hope it works.
If you did everything right, it should work fine.

Step 7: You're Done!

This is the finished product set up in my bedroom.
You turn it on and off with the switch on the PSU.
Don't bother setting the clock or anything, because it loses its memory every time you shut it off