To claify something barristan talked about...
While a resistor is needed when wiring your own LEDs, the assembly from AutoLumination already has one. I think this got lost in the agruing of the other threads.
If you look at the black heat shrink tubing near the assembly, the red wire has a bump in it. That's the resistor. It's been sized for 12 Volts.
I've played with the voltage on my units and if you go much above 12 volts, that resistor gets hot. If this happens, switch your power supply down a notch.
These units are very bright at 12 volts so I'm not sure where his comment about needing a higher voltage comes from. However, as he correctly discuses, if you DO use a higher voltage, you'll need another resistor.
For the low end, when I drop the voltage to about 9 volts, one of the LEDs doesn't light and the other two were dim.