Before someone tells you otherwise, a 6v switch is perfectly fine in a 12v application. A switch is a switch, it doesn't really care what voltage goes through it. But current is another factor, and 6v switches are designed to handle TWICE the current of a similar 12v switch. It's simple math:
P(watts)= V(voltage)* I(current).
Say 60watts(typical headlight load)= 12v x 5amps.
Likewise 60watts= 6v x 10 amps(exactly double the current).
I've got an OE VW Bug 6v headlight switch in my 12v Spyder.