A 12V bulb, powered by 6V will be much dimmer than a 6V bulb of the same wattage.

If you want to improve the tail lights, use plastic lenses and get some 6V led bulbs.

Here is what I did:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=1001454