Sounds like a resistor for when they were still 6 volt lamps.
If lights are 12 volt they should be wired in parellel and no resistor.
Look on lamp for wattage or look up number on lamp, turn them on and measure current to lamp with an amp meter.
Example:
If 40 watt, light should draw about 3 amps each. (I Guessed 40 watt,)
. P .
I * E
P = Watts (40)
I = Amps (3)
E = Volts (13) approx
Hope this helps...
Jeff