Why would your lower brake lights not work when the upper brake light does work?
By the nature of the question I assume that you are referring to
the tail lights and brake lights. My Explorer has four bulbs. Tail,
brake turn and backup. They are all in separate compartments of the
light assembly. So it could be that you have just burned out one
bulb. If you are referring to the upper brake light in the rear
window or at the top of the tailgate, same thing. One or more bulbs
are burned out. I see people driving all the time with both lower
lights burned out and the top working or visa versa.