on the vss (vehicle speed sensor) its a plug harness that is on the back of your tranny, its connected with one bolt. all you have to do is unbolt the one bolt and pull the sensor out, inspect the gear, its prob old. you can get a new one for like 3$ at the dealer i believe, or if you have gears american muscle or any other repuatable mustang supplier can get you the correct tooth gear for whatever rear is in your car.
Will