[drolston]

That is theoretically possible but quite problematic. If you are going to run on 6v, that means you have a 6v generator; how would you charge the 12v battery? How do you isolate the 6v components from 12v while starting? Failure to isolate could burn out bulbs, instruments, radio components, etc. As you can see this idea is way more complicated and expensive than simply converting the whole car to 12v, which has been done thousands of times by people with little or no electrical system knowledge.