Very nice project. I have been considering something similar. I am not so much interested in the audio facilities as some enhancements to travel capability such as GPS navigation assistance with the ability to interact with the Internet for Points Of Interest along the route. As well, I expect to be able to add hands free operation of a Skype telephone capability and internet browsing.
There are a number of Android Apps available that have a piece of hardware associated with them that allows you to tap into the On Board Computer that is available on all modern vehicles. A "dongle" type device plugs into the On Board Diagnostics (OBD) connector located just to the left of the steering column, low down near the hood release. There is a widely accepted standard that nearly all manufacturers conform to regarding pin assignment etc. called OBD II.
The dongle has a Bluetooth connection to the Android device thus making available to the App all data necessary to allow the creation and real time display of a wide range of vehicle instrumentation including things like ignition advance, manifold pressure, oil pressure, RPM, Torque, Temperature, Speed, engine load, efficiency, etc. and even engine diagnostics. The OBD Connector is the same one a mechanic would use to diagnose your engine.
It would also be possible to get current traffic information.
Of course, none of this would prevent having a good audio facility if desired. Even front and rear cameras could be integrated.
I am surprised the auto manufacturers have not gone this route and integrated all of the vehicle standard monitoring and entertainment into a single software controlled display.