Thanks for the info jrcfg,
I agree with EnigmaOne, connect them in series if you can.
Do they all have a common ground i.e. the tractor body or do they have separate +/- terminals, if they have a common ground a voltage regulator might be the only option.
How much power do they use? - to me being heaters it sounds like they're pretty high powered so at 6V a linear regulator would have to dissipate half the power, so a switcher would be a good choice.
Come to think of it if they're heaters then they would behave electrically as resistors, then you might be able to getaway with just pulsing them with 12V at 25% duty cycle to cause them to dissipate the same amount of power as if they were connected to 6V.
Edit: 50% changed to 25% , I remembered Power = V^2/R!