I am building a robot using a small radio flyer wagon. I have two 12 Volt DC motors for power that are connected to the arduino via a motor shield. I'm having trouble executing turns by spinning one wheel in reverse and the other forward. I'm thinking of using a stepper motor to steer the unpowered back wheels. How can I control the existing two motors and a stepper motor. The specifications on the motor shield indicate that it can only control two regular DC motors or one stepper motor. I'm looking for some direction and ideas.