Assuming you have already replaced the bulbs, there is a switch that the brake pedal touches when depressed that activates the lights. I've seen this switch knocked out of place before or even broken.
Here are two links. The first, the guy shows the location and basic overview of the brake switch in a 2004 Expedition. The second, shows an actual replacement for an 2004-2008 F-150.
https://m.youtube.com/watch?v=upj3LnrJd0M
https://m.youtube.com/watch?v=ZfNk947lqYI