Some cars these days don't have a compass always showing. You have to fiddle with the menus only to have it disappear again. I combined an Arduino Pro Mini, an OLED screen, and a magnetometer to get this to work. There's even an Easter Egg that enables zombie apocalypse mode after so many turns.
Please vote for it at Instructables: http://www.instructables.com/id/DIY-Digital-Compass-for-your-car/
The code: DIY Digital Compass Sketch (for your car) http://www.instructables.com/id/DIY-Digital-Compass-for-your-car ยท GitHub