Arduino external power input can regulate 12V down to 5V, but you can get car accessory (cig lighter) USB power/chargers pretty cheap. You can also use a voltage divider to go from 12V to 5V but IIRC car 12V isn't super-stable so perhaps 12V down to 8V and that through a regulator would be best.
There are loads of cool switches/dials at many sites, some with lights of their own. You could go with capacitive touch sensors, zero moving parts and able to hide under dash padding or non-conductive panel but harder to get right than mechanical switches. You could maybe use IR-interrupt or IR-reflection for touchless switching.