If you use only a resistor, you will have to seriously under power the LED to prevent killing it. For design purposes, you should plan on seeing 60V spikes in an automotive electrical system. These are caused by suddenly disconnecting a current source from the charging circuit. Resistive lighting can handle the spike just fine, but LEDs can't. You really need a current-limiting control circuit.
LED's do handle this just fine. The light internal to the window switches in the SDL, for example, are LED's. They'll live for years, much longer than a 'more robust' incandescent will. All they have is a series resistor. LED's are commonly seriously overdriven in multiplexed display operations. Huge current spikes that would fry them if continuous. But they're intermittent, as are the load-dump spikes in an automotive system. For nighttime illumination, the LED is going to be fairly weakly driven anyway. -- Jim
