I'm not sure this is really needed but maybe you could use a baseline voltage divider to set aref. Run a 1M resistor from the car's "12V" to aref and a 470K from there to ground. If you use the same two values for the other voltage dividers that would give you 100% of the range.
I have used 1M/470K voltage dividers to pick up signals from the tach and neutral sensor signals on a motorcycle. These are both off/on sorts of things so I haven't needed to worry about the intermediate voltages.
By the way, does a car have a regulated 12V buss somewhere or does every circuit have to do its own regulation?