Recording Servo/Motor movement with arduino
It sounds like you want to move the car manually and have the Arduino record this movement, then "play it back" later.  

If this is the case, using a rotary encoder (example part) of some sort would be a simple way of doing it. There are various options to detect the rotation and mounting methods, magnetic sensors, optical sensors, etc so have a look around and see what suits your project. Use to sense the amount of rotation, record and then simply tell the stepper to do the same. 

Since the stepper has magnets and windings that will generate pulses on movement, you can also use the stepper itself as an encoder, so you may want to experiment in this direction. Here is a good page on the subject with an example circuit shown below (written by a very knowledgeable and enterprising chap on the Piclist)