Here's what I did with LED's for my bike tail lights, turn signals and license plate light. I used a truck LED unit, modified it and made a billet aluminum frame. The turn signals use standard 1157 red led "bulbs" (2 per side). The license plate light is a billet housing with a custom circuit board. On the subject of resistors, since I used standard 1157 replacement "bulbs", they come with the built-in reisitors for hi-low brightness. The tail light has a circuit board inside that "chops" the incoming voltage to about a 50 to 60% duty cycle, giving the appearance of low brightness. When you apply the brakes, the chop circuit is bypassed, giving the led's their full-on brightness.
Regards,
Steve F