Tom, my 39 buick had two separate 6v motors. One for general heat and one for the defrosters. Both in the same under dash enclosure.

So I wired them in series to use on 12 volt system. Basically for the few times I needed the windshield cleared of moisture or needed a little heat. Depending on your heater you may want to substitute a 12 volt motor. That's if it is a single motor unit.

You are also doing so much you will probably put in a new electrical fuse box. Cars before the fifties did not fuse everything like they did in later years..