To be sure, you should:
* Check your owner's manual: This is the best resource for the exact location in your specific vehicle.
* Look carefully under the dashboard on the passenger side: Feel for a small, square-ish box that might have a button on it.
* Consult a Ford Ranger repair manual: A Haynes or Chilton manual will have diagrams showing the switch's location.
* Search online forums: Searching for "1999 Ford Ranger inertia switch location" on forums dedicated to Ford Rangers might yield images or descriptions from other owners.
Remember, if you find it, be careful when resetting it as fuel may splash.