Purchase a new bulb and see if it works. If it does, the ballast is good, if it does't the ballast is bad. Return the new bulb and purchase a new ballast. Or, just buy a new fixture....its cheaper.
Remove belt and check to see that it rotates freely and smoothly
check fuses & brake lights switch located on brake pedal
check to make sure the belt tensioner is functional if not it needs to be replaced
How do you know if it needs to be replaced
First check the fusible box make sure there not burn. If all of them are good. Check the bulbs probably there all burnout or the flushing. Box needs to be replaced that one is also in the fusible box
Check the sender in the fuel tank.
One of the air bags probably has a leak.
There is a spring loaded tensioner that is supposed to keep the serpentine tight. If it is loose, it needs to be replaced.
Typical Ford wiper switch malfunction. The wiper switch needs to be replaced.
Evidently the bulb is blown out. It needs to be replaced.
PCV valve needs to be replaced, or air filter needs to be replaced, or EGR system needs to be cleaned out. If its the EGR system your check engine light will be on.In other words, it's time for a tune up.
The gas cap is loose and needs to be replaced.