100 ohm resistors.. It depends on the light If it is an LED perhaps 200 ohms would work, or 300 more than likely, but if it's an incasedent,
Well a 12 watt lamp draws two amps at six volts, so you'd need a 3 ohm resistor, and it would have to be a 12 watt.. That is a rather heavy wire wound power dropper.
and if the lamp wattage is higher (and it likely is) the resistance goes down and the wattage goes UP (Wattage = or greater than lamp wattage,,,, Resistance = 36/Lamp wattage or for safety 49/lamp wattage (7 volt drop))
P = I/r I= E/r P=e^2/r r=e^2/P (e=6 or 7 (Volts you need to drop), P = Lamp wattage, R is the value of the resistor)
And remember the resistor needs to drop at least the same wattage as the lamp.