I just recently finished my CarPC setup including the replacement of my stock speakers (2006 RAV4). However, I used a laptop instead of a PC because of mobility and relatively ease for updates.

Here is where you can find detailed of what I have done.
[http://www.gruporav4.com/viewtopic.php?t=401

You could also go to www.mp3car.com if you want further info on your planned setup. I've done most of my research here and was really amazed on the tons of info found.