I have similar project on my mind =)
What model you have? I have 156 Sportwagon.
For the radio, this can be bought from mp3store but for us here in "old continent":
At my previouse project I used Silabs USB radio:
but this is not very sensitive and only have thin wire as antenna. I can be connected to decent antenna if you solder a good connector to it.
Where did you plan to place the PC? I intend to use the small place where the CD-changer is now. Allso idea is to keep stock speakers (for now) and standard wiring too if possible...