To be certain, you should:
* Consult your owner's manual: This is the best resource for the precise location of all modules in your vehicle.
* Look for a large, black rectangular box with numerous connectors: This is the most likely candidate.
* Use a wiring diagram: A wiring diagram will show the location of the PCM and its connectors. You can usually find these diagrams online (search "1998 Ford Explorer Sport wiring diagram").
Keep in mind that while the PCM is the primary computer, other smaller control modules exist for various systems (like the body control module). If you're looking for a specific module, be as specific as possible in your search.