Go to radio shack and ask for a wiring harness for your car stereo.
This is a harness to plugs in the the harness off the back of your radio. It also illimenates the need for a wiring diagram, cause all the wires an the wiring harness are listed. 12v+ ign., 12v const., Ground are really all the ones you need. If your new radio supports it, you might want to hook up illumination as well. This will make the radio dim it's lights when you turn on your head lights.