Because its location isn't completely standardized, the best approach is to:
1. Consult your owner's manual: This is the most reliable source for the location in your specific vehicle.
2. Look under the dash on the passenger side: This is the most common area. Focus your search near the fuse box and other electrical components.
3. Search online: A quick Google image search for "1998 Ford Windstar inertia switch location" might reveal pictures of its location in similar models.
Remember to be careful when searching under the dash; avoid touching anything you're not familiar with.