I was snooping around today and happened to run across this site. It may be very helpful for anyone wanting to work on some LED related projects. This site will compute the current limiting resistor required. All you need to do is provide your source voltage (the operating voltage of your vehicle, ~14V), the typical forward current and forward voltage of the LED. The latter two values are specific to each individual LED, dependant on their manufacturing specs. 1.9 - 2.1VDC are common along with a 20mA forward current (although there is a wide variance so use values specific to your product).
There is also link on the page if you plan to build a multiple LED array. This will be most helpful if you are connecting two or more LED'* is series.
Remember, LED'* are polarity sensitive and won't illuminate if connected backwards.
Hope this link proves helpful.