326 man
I dont know anything about your car, but I may be able to help with the voltage verse wattage stuff
The formula is wattage = volts x current
so 6W = 6V x 1A, 12W = 6V x 2A, as it is linear that means 6W = 12V x 0.5A, and 12W = 12V x 1A.
Solder sniffers like me remember this formula as IVY WATTS
As you can see, if you changed to 12 volts, you would get the same wattage for half the current, as you have twice the voltage
Now, car wiring is amongst the poorest I have ever seen, they get away with this as the international lethal voltages, are, or were when I was a boy, 50 Vac, and 30 Vdc, your car is dc.
At these voltage levels wiring is specified in current rating, relays, switches etc tend to have both voltage and current rating.
So if you just change your lamps for higher wattage ones, make sure your cables,relays, switches etc can handle this, I have seen friends get into trouble just uprating from 65/55 to 100/80 lamps
If you use the above formula on ac circuits, or any of OHMS LAW, please remember to use the RMS and not peak values.
My advice, for what it is worth would be to change to 12 volts, if you have anything that must be operated on 6 volts, this level is quite easy to achieve just using two resistors as a potential divider, if you have that problem, e-mail me, and I'll get the details off you so I can work out the ohmic value and wattage for you