Buddha,
If you have a speed control for the blower, you already have a rheostat.
That is, is you set the control to 1/2 and have a 12 volt system, the motor will run at approx. the same speed as the control set to full on a 6 volt system.
Somthing to consider, DC motors have a really wide operating range. Although if you run the motor at 12 volts on FULL, check to see how hot your motor is getting.
