Electricity 101: 3rd Brake Light

[29er]

I've been watching ebay for vintage brake lights, thinking about adding one as a 3rd brake light to my coupe, which has a 6 volt system. But many of the lights for sale are 12 volt.

Knowing very little about the miracle of electricity , I wonder whether these 12 volts lights can be made to work with my 6 volt system .

Does anyone know of a detailed description for boneheads like me of how to convert a 12 volt light to a 6 volt light?

Mike in Oregon