It's a small, cylindrical component usually attached directly to the top or side of the starter motor. It's often identifiable by its large electrical cable connections.
However, because there are variations depending on engine type (4.0L V6, etc.), it's best to consult a repair manual specific to your 1991 Ford Explorer's engine. A Haynes or Chilton manual will provide diagrams and detailed locations. You can also find some diagrams online, but always double-check against your specific vehicle.
