Krowten-Dog posted a useful tip on this a while back. There is a system menu and there are a number of things you can adjust with it, including the bluetooth volume (which I found useful):
1) Turn the car to "ON" or "ACCESSORY"
2) Turn Audio off by pressing the volume knob in (only necessary if it is on)
3)Press and hold the SETTINGS button
(If the safety acknowledgment screen comes up, press enter to get past it, then press and hold the SETTINGS button)
4) While still holding down the SETTINGS button, turn VOLUME KNOB approximately 20 clicks in either direction but do not press the knob (i.e. don't turn on the audio)
This gives you several menus for checking a variety of diagnostic functions, including a system self-test. You can also reset and control a number of bluetooth, speaking and other features.
Hope that helps...
