Of course it is all possible but it is not at the level of just works for most people.
"You think that would be good, but in reality it would be annoying. It's nice to have that stuff have its own controls, particularly a volume knob.
Volume buttons are the devil."
So wouldn't it be cool if Android could read the volume knob and even use it and the other buttons that the car uses?
"In both the GPS and OBD-II cases, you can bet that the manufacturer would charge you more than you'd pay to get the functionality from a third party."
To start with probably. Eventually it will just be standard equipment just like traction control, and ABS is now.