This might be old news but i post it if this can help anybody

On my 2000 Blazer ZR2 the the left taillight did not work properly
When I stepped on the brake pedal the brake light did not come on and the taillight stopped working/the light bulb for the taillight went dead

I figured that this was a ground problem so i cut the ground wire goint to the taillight/brakelight socket holder and wired the ground straight to ground and that made the taillight and brake light work as it is supposed to do

I have a spare light bulb holder for the taillight/brakelight so i checked out how it is put together and after looking at the socket holder i can say that there is no reason to cut any wires like i did.

If your taillights does not work as they are supposed to just take off the taillight and simply pull the ground wire out because the only thing that hold the wire in place is just a little "hook"
The problem on my car was that the ground wire contact surface in the socket holder was corroded and dirty so all i had to do was to clean it

If i had done this the first time i would not have had to cut the ground wire and also solder the wire ends back together

On the first picture to the left you can see the taillight and socket holder.
You have to pry off the blue plastic before pulling the ground wire straight out

On the right side picture you can see the "hook" that usual are corroded and dirty that you need to clean to fix the taillight problem

The "hook" on the wire is what holds the wire in place on the socket holder on both for ground, positive

So if you have any problems with the taillights i would clean the hooks that goes into the light bulb socket holder and push it back in

You might have to use a little screwdriver to get the "hook" at the end of the wires back in after cleaning

Remember to put the sealing you can see on the picture to the righ back into the socket holder and also put the blue plastic clip back on

 

What year did they change the bulb setup? My 04 only has 3 bulbs in the taillights, two for tail/stop/turn signal and one for reverse...