Found this:

uhlek.com/stereojack.htm

Pretty much accurate. Tap into the four wires there, red/black and white/grey (or red/grey and white/black, I can't remember, but they're twisted together) and it'll feed right in. You still need to cut off your CD changer because you need to have a cd playing to work, but if you never use your player its a great solution.

Blitzsafe makes an adapter that will give you a set of RCAs and let you keep your cd changer.