Ballast resistor for lights?

That isn't as easy as you make it sound.

Since the lights normally ground to the tractor's frame, you would first have to insulate the first light's mounting point completely from the tractor. No metal to metal contact whatsoever.

That first light would have to then ground thru the second light.

At that point, you would have run a wire from the first lights "metal base" over to the second light's 'hot' terminal to have the two in true series fashion.

If you just run a hot wire from one light over to the other, you are actually still in parallel and dropping 12 volts to each light because those lights are grounding at their mount.

Like Bob says, buy some bulbs.

Just my view,

Allan

Unless your C is a late model with seal beams, the lamps use standard single contact bayonet bulbs. Replace them with #1156 12 volt backup light bulbs. (A box of 10 will cost about $7 or so.)

Now as you discovered you can burn a 6 volt lamp on 12 volts. However it WILL have a very short life on 12 volts, either blackening or burning out completely within a minute or two.

One last item to consider: If you burn out one lamp, all your calculations will be wrong for the remaining lamp. It will get a higher voltage at that point, possibly burning out your remaining lamp.