Why would my head lights only shut off and on every 20 seconds
Headlights in most cars have a circuit-breaker, as opposed to a fuse, so if there is a short in the wiring your headlights will not suddenly fail completely as with a blown fuse. The breaker heats up, shuts off the lights to prevent an electrical fire, then re-sets itself. I suggest (if you are not a do-it-your-self'er) take it to a mechanic to trace what appears to be a short.
Search Ford F-150 Questions
Ford F-150 Experts