Most any LED will do, biggest decision will be what size and mounting style you want to utilize. There are panel mount types at RS also.Originally Posted by wkolodny
Their part # 276-068 for example, has a forward voltage of about 1.8V.
~13.8V-1.8V = ~12V the resistor must drop.
This LED will tolerate 20mA, so the formula to determine optimal resistor value for maximum brightness is 12V/20mA = 600 Ohms. Resistors aren't readily available in that value, and lower value = higher current and is less tolerable so next common value of 670 Ohms is about right. A 1K would work fine, but be a bit less bright (12V/1K=12mA). Less bright might be a good thing depending on how you mount it.
I'd recommend experimentation with several types to find the right orientation and brightness. There's nothing more annoying than a "laser beam" lookin' LED in the eye when you're driving.
Don't forget to spec the power dissipation of the limiting resistor. 20mA X 12V = 240mW. This would be tolerable for a 1/4 watt resistor indefinitely. If your chosen LED is much brighter and requires more current, you may have to go to 1/2 Watt resistor.