Grab an Arduino Uno. Add a 7805 5V voltage regulator to run it. Also add a 5v micro relay for the output for your light. Then just two more resistors, one 1K the other 4K to give you a 5:1 voltage reduction and use that as your input.
Program the Arduino to double wink the volt light when you first turn the key on as a nice touch.
Program the Arduino to light the light at under 13v or over 15v
Program the Arduino to flash SOS in Morse Code on the light at under 12v or over 16vhttp://www.ebay.com/itm/UNO...&hash=item3cd0d45931
You can mount the circuitry on an arduino development shield
[This message has been edited by phonedawgz (edited 09-08-2013).]