12v voltage regulator

The proper way to do it is to not worry about voltage at all.

Calculate the current your LEDs will need based off of a 14.4vdc power supply (worst case condition).  The resistor you will then be good up until 14.4.  Anything above that and you'll be pushing the limits of the LED, but unless something major happens you should be alright.

The LEDs will still work with the engine off, they just won't be as bright.  Depending on the LED you might not even notice the difference.

Example:

If you're looking for 10mA at 14.4vdc you would use 1440 ohms

at 12vdc flat you would still have 8.3mA to power the LEDs, which should be more then enough (although over a 10% difference).

Technically, a 12vdc regulator wouldn't work for your application anyway.  To maintain the voltage you need to keep the 12vdc regulators input voltate 1-2 volts higher then the regulated output.  If you input 12vdc to a 7812 you will NOT output 12vdc, it will be closer to 10vdc.

To fix this, select a 7805 and run all your LEDs at 5vdc if you are really worried about these small differences.

Run them in series and calculate the resitor value so that each led will have about 30ma. That way they will be a little overdriven at 14.4v and just above 20ma at 12v. And don't worrie about cooking them at 30ma, they will be fine.

If you want ps me and I'll send you a little program that let's you calculate the resistor value based on the amount of led's, desired current and foward voltage.