Since you are getting a (+)volts between 6V-15V you have to bring this down to a 5V level for sensing with a digital input. There is no need to use an analog input pin for this. Connect the lightbulb-transistor node through a 10k series resistor to an Arduino digital pin. Then, add a 5V zener diode between this pin and ground (cathode at digital pin, anode at ground).
It might be easier to measure the voltage at the transistor base. Where is this coming from and what is the voltage?
The Rugged Motor Driver
: two H-bridges, more power than an L298, fully protected