To find it, you'll likely need to:
1. Consult your owner's manual: This is the best place to start. The manual will show you the exact location for your specific year and model.
2. Look under the driver's side dash: Feel around for a small, usually square or rectangular, push-button switch. It might be near the fuse box, or tucked away in a less obvious location. Sometimes a small cover needs to be removed.
3. Search online for your specific year: Search "[Your Year] Ford F-150 inertia switch location" on Google or YouTube. Videos often show the location clearly.
Without knowing the year of your F-150, I can't give a more precise answer. The owner's manual is your best resource.