I have a 2002 GMC Envoy and the third brake light how do you replace this bulb?

I think you will find that it is a LED light strip assy. that you will need to replace...not a standard light bulb. Here is the procedure from the repair manual to remove it. 1.Open the liftgate. 2.Lower the rear portion of the headliner by releasing the 2 headliner retaining pins. 3.Disconnect the high mount stoplamp electrical connector (2). 4.Remove the grommet (1) from the body NOTE: Do NOT raise the liftgate while the liftgate glass is open. This can result in damage to the high-mounted stoplamp, the liftgate hinges, or the liftgate glass. 5.Close the liftgate. 6.Open the liftgate window. 7.Remove the 2 screws that retain the high mount stoplamp to the lift gate window reveal molding. 8.Remove the high mount stoplamp from the vehicle.

Agree with milesauto on replacement procedure. Also i think the assem. is aval. aftermarket, check parts stores. Pretty high dollar @ dealer!