Check out this page and see if it helps...
http://www.kpsec.freeuk.com/components/led.htm
Basically, LEDs are short circuits, and will destroy themselves, and/or the power source in short order if you don't add resistance in series with them. The formula lower on the page will tell you what value resistor you need. Unless you are on a regulated line, assume car voltage is 13.7 volts I think.