Custom LED taillights

well im thinkin of making custom leds for the taillights for a da i was going to pull the light apart and make a bracket and so it will have led’s all across the back and also in the blinkers. my question is how do i find out what resistors i need to dim the leds from 12v to arond 7’ish so i can have dim running lights and brite brake lights. if you can help me out that will great.
thanks
justin

http://lmgtfy.com/?q=12v+resistor+calculator

i have been thinking of the same thing. but i;m taking the easy way out and using existing tech ie: semi truck leds. they have various sizes and lengths . but i have been alittle to lazy also i want to find another set of tails to butcher before i do my own.