https://www.classiccarstereos.com/6v-12v-converters.html (6v neg to 6v neg)

https://www.classiccarstereos.com/positive-ground-6v-to-12v-converter.html (6v pos to 12v neg)

Those are sold by many venders - check around for price.

Maybe someone has used on of these?
Says it will work with positive ground.