Here's what I've done with mine. I don't have them installed yet; been working on my engine swap.
LMC and Dakota Digital both make an LED tail light conversion kit. But, I wanted something a little bit brighter and a little bit more spread out over the area of the tail light. Plus, the aftermarket ones are on a single circuit board. LEDs don't have a very wide viewing angle of light, so this will be spread over four circuit boards that will be at about 20 degrees from each other as to curve with the tail light lense.
Here's kind of what I had in mind. I'll have one circuit of 6 (5 in the middle on then one on the side) for tail lights, one circuit of 31 for brake light/turn signal and then one circuit of 12 for the back up lights.
Here's the mock-up of the center 5 LEDs. I built a voltage regulator so that as long as the vehicle is putting out between 7 and 30 volts, the LED circuit will have 5 volts avaliable. That way, the output will be the same whether the vehicle is off and there is only 12.6 volts or the vehicle is running and there is 14.5 volts.
Here's the circuit connected to a car battery. I wasn't sure how bright it would be, but they're damn bright.
Here's an idea of what it will look like with the lense over it and the lights out.
