> But your entertainment system is the screen where the rear-view backup camera gets displayed.
Personally I have learned to turn my head.
> You need the computer controlling the transmission to be able to tell the computer controlling the entertainment system to start displaying the camera feed.
You can have data that goes one way.
For example it's very common in industrial applications dealing with potentially high voltage to use 'light connectors' to join disparate electrical systems. Basically you just have some infrared transmitters on one side and a infrared sensor on another and thus you can transfer information without a direct electrical connection.
So it's very possible to have a properly functioning gauges and other devices without the ability for any attacker, no matter how determined or skilled, to use your entertainment system to subvert your automobile remotely.
> And I think you'll find that by the time you hit every use case (safety interlocks that prevent changing GPS coordinates while the car is driving,
Idiotic safety controls. If I had something like that on my car I would just turn the GPS off and use my cell phone and google maps, or other equivalent. I don't need anti-features in my car. Driving is hard enough without having to fight my car for control.
> vehicular speed being to augment the GPS in tunnels, etc) you'll find that just about everything gets connected somehow.