The 3rd stop light bar at the top above the window isn't working. Where do I start? is this an easy fix? It's a 04,4WD, 4Dr Blazer. Are these LED lights?

 

Its probably just a bulb. The bulb type/number is 577. However, the Haynes repair manual says it is a "special bulb" for 4dr models and must be replaced by a dealer. I'd pull the bulb and see if you can match it at the local parts store. If not you may have to go to the dealer to get it. Or this may be a good time to make up some LEDs that could work.