To be sure, you should:
1. Consult your owner's manual: This is the best place to find the precise location.
2. Look carefully under the dash: Feel around for a small, usually square or rectangular, box with a red button. This button is what resets the switch.
3. Check online forums: Search for "1999 Ford F-150 inertia switch location" on forums like F150online.com or others dedicated to Ford trucks. You might find pictures from other owners that show its exact location in a similar model.
If you can't locate it after searching these areas, it's possible your truck doesn't have one, or it might be in a less common location. A mechanic would be able to assist if you can't find it yourself.