I think physical buttons are better anyways and I do not like touch screen. I also think physical buttons are better than voice controls, too. This would be applies to other computers too though, not only the car.

We have computer with a full keyboard, that all of the commands can be entered, and many key combinations are possible, so a touch screen is not needed. Even many function should not need mouse but sometimes mouse is helpful, though.

Specifically in a car controls (I am sometimes passenger, not the driver), tries to lock some controls while car is moving, preventing the passenger from adjusting the controls. It is better to allow the passenger to adjust the controls in order that the driver will not be distracted from driving the car, I think.

I think that a reasonable design for buttons will including a numeric keypad. You can include other controls such as volume, channel, play/pause/stop/rewind/fast-forward/previous-track/next-track/eject, radio (AM/FM), and possibly a few other function if needed, but many function can be done by combination of other function, e.g. sequence of numbers for a more complicated function option