Our friend Seth has sent us some information about his fantastic project using SECUduino.
Using a Secuduino, a RBBB (Arduino clone), and a Belkin bluetooth stereo receiver, I have been able to successfully emulate a CD changer for old generation Saab 9-3's (MY98-02). Using a short amount of code, I am able to tie into the car's I-Bus (instrumentation bus), which communicates at 47.619kbps. I simply send a CANBUS id and command which the car's head unit interprets as a married CD changer. When I select the "CD changer" on the radio, it simply turns on the CD changer Left and Right audio channels. I have the bluetooth receiver tied to these lines, so now whatever audio I have playing via the bluetooth device will now play through the car's audio system.
You can find more information http://bluesaab.blogspot.com/