Check out this article called PID Without a PhD
http://www.embedded.com/2000/0010/0010feat3.htm
Also a recent avrfreaks.net post has a PID control library
It is very unlikely that a small step motor will be able to operate the throttle of a car.
One of the several sub problems is to make a test rig to open an close the throttle with engine off.
Test the throttle with a fish scale to determine the force needed. That way you can be sure.
Also consider a DC motor with a pot for feedback.
Perhaps you can tap into the TPS which is a pot on the throttle shaft used by the Engine control computer.
Many older cruise units use engine vacuum to pull a cylinder for the throttle. Perhaps a junkyard trip is in order
If you keep the gear ratio low on the actuating motor it will return to idle by just removing power. Find a motor that you can spin the shaft by hand and test to see if the throttle return spring will back drive it.
Good luck