If you're going to be using a ton of LED'* and don't want to keep implementing resistors, you might look into purchasing a variable voltage regulator. They're only a couple bucks, and then when it'* hooked up, you're done! At most it would take 4 components if you use the capacitors like they suggest (which aren't required).
If you like that idea, I'd recommend using the LM317 by national semiconductors (http://www.national.com
). You could limit the voltage to exactly 3v (or less), or even 6v if you want to arrange to hook 2 LED'* in series between the power rails. You can buy these at local electronic parts stores, and even check radio shack to see what they have. Push come to shove...use http://www.jameco.com
to order. The reason I didn't use these in my car was because of space requirements.
I used 5000-6000 mcd LEDs. That gave me the about the best amount of light, without being too dim, nor too bright for single LED applications inside the car.