I have a 2005 GMC Envoy. Have tail lites but no brake lites. Third lite on top works whet brakes applied

Yes. See this post on the single versus double filament bulbs
Brake light doesnt work Ricks Free Auto Repair Advice

You may have a ground problem? The ground wire is black, you could try a redundant ground, see if it helps? You could also use a test lite and check voltage at applicable lamp, with turn or brake lites on. Your lower brake lites and turn signals should use the same bulb filament, just not at the same time. If I'm correct, your lower brake lites are wired through the turn signal switch, voltage.

The tail light fuse is under the rear seat on the drivers side.

Check your brake lite bulb socket alot of times even a newer car will go bad from heat ing every time bulb comes on,,also check for grd on that socket, also should check brake lite swich on brake peddle

Check your brake light switch located under the dash on the top of your brake pedal,also it could be a broken tail or brake lite bulb.

i have a 95 gmc suburban that keeps blowing the fuse when break is aplied i even disconected the back lights and it does the same what could it be

NO YOU HAVE A TAIL LITE OUR OUT. CHECK YOUR CRUISE CONTROL IF ITS NOT WORKING IT'S THIRD BRAKE LITE.......

Hi Ahunley, Sounds like you have cross the connection wires to the unit ( positive to earth & earth to positive) Try swapping the connectors over.

Hope this works.
kind regards
David

some one has the bulb in backwords or check the wires they might have them crossed