Does your 6v heater have a 'speed' control? Most Chevy heaters had a 3 speed, or a variable control.
If you do, then you actually already have a voltage reducer. I'd guess that a 12v supply to a 6v heater motor running at 1/2 speed has the same rpm as 6v supply to a 6v heater motor running at full speed.
I'm not sure this is exact, but DC motors in general have a really wide latitude in supply voltage. Of course, running a 6v motor at 12v high speed will run hotter than with a 6v supply.
Personally, I'd run the 6v motor on high speed for 5 minutes or so (with the motor cold and not running) - then see how hot the motor is actually getting. Any potential damage to the motor in this situation will likely be from heat - melting solder, etc.
If you do this, make sure the heater motor has an appropriate fuse for safety in case of motor failure.
