Adding a steering servo
If the motor you are using has only two wires, and as you described it's response to the direct connection to the power, it can't be a servo. The motor is a normal dc motor.
If you want to control the amount of the turns on your rc car you should try to play with the pwm signal, I don't know if it will work or not but at least you will have tried. And remember not to connect the motor directly on the micro's pin if there no motor pins on your red board.