What would cause the third brake light on a 2003 Chevy Tahoe not to work when all other lights work and how would you replace?

I have same year Tahoe and same problem. Check fuses first to make sure it isn't a fuse. The third brake light assembly can be removed by opening the back door and unscrewing the screws. Then unplug the connector. A new unit is about $175.00 and the whole unit must be replacee as it is LED light not bulbs.