6 volts = 0.065 amps * resistance
resistance = 92.3 ohms
(12-6)/0.065
So if you bought a 6 volt LED, and a resistor that was as close to 92 ohms as possible, it would make the bulb last running at 12v. There would be a peak of 14v but I would think that an LED could take the surge better than the little incandescent ones cars used to have. To be on the safe side you could use a 123.1 ohm resistor which would protect against the 14v surging, however when running at 12v the light would be a bit dimmer