Not so simple question. It really depends on how many leds you have, what kind of leds, what is led's voltage drop and current rating. You also have to calculate needed resistance to not to burn the leds with excessive current. Cars voltage isn't very stable, so good rule is to use 14.5V instead of 12V when calculating things. There are also some "plug-and-play" sets for car use, a bit easier to install. Some basics: http://www.theledlight.com/LED101.html Leave it to someone who has done these before, if you don't know what you are going to do. It saves time and all the hassle ;-)