well, it depends on many factors:
The most importarnt is to know if the circuit will be powered with no key ignition on. In other way, if the circuit is to be powered with car battery directly. If this is correct then you most convenience aproach is to use a DC/DC down converter, due it high efficiency
if not, then you can use many types of circuits depending of the current you need and precision of your A/D, if you need low precision and low current you can use a simple zener diode. If you need much current and precission you will need linear regulators or specific regulator with more precission.
i'll hope you be usefull