Almost all LED's work off of 5 volts or less. There is just a resistor placed in series with the diode to adjust the supply voltage down to what is required for each individual diode. The only difference between 6 volt and 12 volt applications is the size of the resistor. So, with a little knowledge of Ohm's law, you can easily swap a 12 volt led to a 6 volt by just changing the resistor.
I have an all red six volt tail light / brake lights built into the taillite.
In the past I have tried the 6 volt RED replacement taillite by BRIGHT LEDs which worked well. The white ones are so bright they look white through the lens.