Quite a current is drawn by those motors. Hard to drop 12 down to six volts without being wasteful at those type of current draws.

Any form of 12 to 6 volt converters in that current range may be expensive as well. Although a string of cheap fifty amp diodes would do it but they will generate some heat. Yet still very cheap and will work well.

I might identify the motors frame number and see if I could find a twelve volt motor. With the same motor frame type as a reasonable alternative.
At least have a friend or yourself do a current draw check at six volts to find the actual draw of that motor.

The rheostat fan speed control burst into flame on my 1954 Plymouth a lot of years ago while I was driving along. I pulled over beside a mud puddle and the two young ladies in the front seat with me jumped out of the car into the mud with their high heel shoes on. I had turned the control off by then and the flames where gone but there still was some residual smoke. I guess from the female perspective the girls though the car was going to explode or self incinerate.

I suspect without knowing that most 12-6 volt decent switching type converters are not easily found or cheap at the current range heater motors draw. Not a big issue in context but just a detail. The heater box below the dash on the firewall seems to not interfere with anything in my case either.