Anyone figure this out as I came home last night with my wife' truck doing this exact thing

 

I followed a pickup this morning (GM) that the 3rd brake light was flashing with the right turn signal. It was confusing as ____. lol

 

So, this is my first time putting anything on here. I have/ had the same issue. I took what you guys said here and messed with the plug. Peeled back a bunch of the tape and found nothing. I pulled at some of the wires and found one of them a little loose in the plug head. Plugged it back in and lights went out. And they still work when brake pedal accuates. Also, its my wife's 09 Suburban. Hope this fix stays! Thanks for the info here!

 

I bought my 07 Suburban LT a few months back and didn't noticed it was missing the top bulbs on the taillights, I have the same issue with the brake light staying on only on the taillight, third light on top of tailgate works as it should, took it to the dealer and they told me it was the engine compartment fuse box, and of course they charged me almost $400 just to tell me that, so I bought a new one and replace it myself, and still the same, I tried everything you guys suggested and the only was they turn off is either by unplugging the fuse at the side of the dashboard or unplugging the connector at the BCM, by the way, the light bulb has two filaments, brake light and parking light and they both stay on, would it be fair to say is the BCM? thanks in advance.

 

Bad grounds will do the same as both filaments can light up through feedback from a poor ground.
Poor grounds are the most common problem.

 

By any chance anybody has a wiring color chart to able to follow the ground wire from the tail light to the BCM before I attempt to replace the BCM? thanks

 

I agree on the bad grounds, they can cause the weirdest symptoms.
According to my 2011 manuals the ground wires (black) for the tail lights go to the left frame rail, behind the left rear tire. Look on the left side of the frame. Good luck

 

I checked the ground cables coming from both tail lights and everything is ok, also based on the way they are wired I hardly think that could be the issue, I followed the ground cable from the frame up to the taillights and the same cable works for every other bulb, comes in on the first one and back out to the next until the last one on top which is the one I have issues with and also there are separate grounds for each side which also eliminates my issue since this problem is on both sides, so at this point does anybody think it should be safe to replace the BCM ?

 

Use an electronics cleaner / lubricant in the BCM connectors first. Fretting corrosion in GM connectors is common before replacing a BCM. BCM most likely will need programming.

 

Which fuse are you removing to get the lights turned OFF ?
Do the lights stay on when the headlights are turned OFF?
What happens when you unplug the PRK LAMP relay in the underhood fuse block?