[MikeK]

Adding a resistor to a 6V motor for 12V operation is not a conversion. It is a dirty fix that converts an amount of energy equal to the motor draw into heat. A true conversion would involve rewinding the field coils so no resistor is needed. The armature remains unchanged. After this is done you will need only half the amperage a 6V+resistor setup will draw. No relay will be needed. No need to put a resistor that will get smokin' hot during a parade under a flammable seat. There have been several write-ups on how to do this.