I think a voltage divider resitor is a bad solution.
The impedance of the motor will be very different in the star or wen you chage the mechanical load.

a) put an 7806 serially in 12V and drive the motor form here (i supose that it is an smaller motor)

b) Do a PWM and do a modulation from 0% to 50% then you get 6V and a fine control of the motor.

I strongly recommend the second option.