My rear brake lights are working but my center mounted light is not. I replaced

high mount stop light(3rd brake light)is on a different circuit than the 2 rear ones(left and right) check the bulb.check your owners manual for fuse location and test it with a test light.also check the 3rd brake light socket(it may be melted/corroded from heat)

If the suggestions that stevieray1 suggested are more than you want to tackle, the best solution is to find a qualified, local repair shop to help you perform the electrical testing that may be required.