To find it:
1. Locate the underhood fuse box: This is usually a large black box near the battery.
2. Consult your owner's manual: This is the most reliable source. The owner's manual will have a fuse box diagram showing the location of each fuse, including the one for the power outlet. The diagram will usually label it as "Power Outlet," "Cigarette Lighter," or something similar.
Without your owner's manual, I can't give you the exact fuse number. If you don't have the manual, you can often find a PDF version online by searching "[Your Year] Ford F150 owner's manual" on a search engine.