[dcb]

I added this to it's own thread cuz it is a neat idea and not iduino specific.

This is entirely doable, but you have to tap into your instrument panel lights, then figure out how big a resistor you need to drop the voltage down to 4.3 volts max (I think) on the backlight. I would assume a system voltage of 14 volts. Use 9.7 volts divided by the max backlight current (from your LCD datasheet) in amps, to figure out the minimum sized resistor needed.

Then unhook the existing connection to LCD 15 and hook up your resistor instead which leads to the light circuit that is controlled by the dashboard potentiometer.