Arduino analog in with 12v
I would like to measure the voltage on either side of each of the
  lights with the arduino's analog input pins, but the arduino's max
  voltage is 5v.


To linearly reduce a higher voltage to a lower voltage requires a resistor potential divider like this: -



So if the bottom resistor is (say) 5 kohms and the top resistor is (say) 7 kohms then 12 volts in becomes 5 volts out but, you might want to give a little headroom so that 15 volts in becomes 5 volts out. so use a 5 kohm for Rbottom and a 10 kohm for Rtop.

I would also be tempted to put 100 nF across the lower resistor so that the ADC sampling operates accurately.