Put the car in park, pull up the ebrake. Tap the DISP button till it shows the real time MPG indicator with the bar. Press and hold DISP for like 45 seconds. THe screen switches over to the dealer settings and such. Change the gas mpg reset to fill up, button or manual. Foot well light to be ON with doors or ON with lights (it goes on with the door opening still). This is also where you change the weather from Cold to Hot and from Metric to English.
I stumbled onto this when trying to reset the mpg meter. I haven't played around with other screens but it's possible we could find another easteregg.