Generally switches, like any electrical device, are rated for the amperage (heat) they can handle. 6v items carry twice the amperage as do 12v items of the same wattage. Therefore all your switch connections will handle 12v just fine given the same load (watts). Only if you increase the 12v wattage consumption (such as a stereo or amp) to over double the original consumption will you overload your original switch or wire. The formula is simple as pie P=IxE
Wattage(P) = Amps (I) times Volts(E)
24 watt taillight = 6 volts times 4 amps
24 watt taillight = 12 volts times 2 amps
If you are considering fixed resistance the opposite applies. Amps will double with voltage.
Sorry if this reply is winded.
