Its been such a long time since I have posted on here...anyways, I have this 12v electric motor that I took from an electric scooter, and I was wondering how I can use a regular servo to control the steering of the motor.
It sounds like you want to drive the steering rods using this motor as a servo.
A motor doesn't normally steer. For a motor to steer, you need to know where it is, and where you want it to be. (Servos you buy contain this system built-in.)
This means you'll need an encoder for the motor you use for steering, and a microcontroller that reads the sensor and drives the H-bridge to keep the motor turning to the place you want.
The Sabretooth controller does not have the necessary feedback to do what you need.
Do you know of any dc controllers that can supply around 5-10amps under $30?
Assuming you can find a suitable encoder for motor position, you may be able to use a "simple motor controller" from Pololu to get where you need to go: http://www.pololu.com/catalog/product/1372
It's $34. I don't think it does the encoder feedback part without you adding a small microcontroller of some sort (Arduino, ATmega, or whatever.)