How do you the replace third brake light in a year 2000 Chevy Blazer?

take off face plate and change the blub.

well, since there are LEDs in the light bar on the roof or on the glass its hard to change the "bulb". here is what i have found. i have a 2002 blazer with the glass mount LED bar. dealership said it would be $230 to fix and replace. i went to a local junk yard picked one up for $35. after removing mine (2 screws on both sides, gently pry up, it will be tough with the seal underneath). the panel that holds the rear cargo dome light will have to be removed, there are about 8 pressure clips holding it in. just pull down and it will come out. the wiring harness is spliced off of the dome light. i may have foam rapped around it. fallow the blue and white wires to connector that connects to black and yellow wires. you'll need a small flat head screwdriver to separate the harness's. on the glass mounted lights there was a plug that went into the body. after about a half hour of my fingers and a screwdriver to pull it out. (hint: attach a string or another wire to fish everything back through the same holes) Gently pull the wires through the back of the vehicle. if you have a new one, reverse the steps that you have taken now. OR if you are trying to find the problem. pay close attention to the rubber plug. chance's are, that the wires were too short, the plug was molded around the wires or have melted around the wires. opening and closing the back hatch may have caused the wire to stretch and break inside the rubber plug. you can cut the rubber plug off (hint:if you cut the plug, make only 1 cut down the side to reuse the plug after fix), fix the wires your self, you may or may not have to extend the wires, i didn't. i recommend soldering the wires together. some electrical tape, and heat shrink it all together. cut the center hole in the rubber plug bigger to fit the wires into. slid the wires back in the plug. then reverse the steps you took to remove it all. hope this will help.