I'm deep into it now. Am about to get a long stick and press the brake looking for 12v applied. While I am in the trunk, I have removed the right side felt interior panel to allow me to remove the Styrofoam under the rear flat panel to inspect for any water or signs thereof. I am happy to report that I saw no signs of water intrusion and inside a soft foam box I saw a large electronic component that was inside of a ziplock food storage bag as well as a cigarette pack sized component in the fender well area near the rear side marker light. If I ever get these interior pieces back into their proper locations, I'll be amazed. The third brake light assembly in the coupe hatchback is now accessible to remove and perform a voltage test. Wish me luck.

 

I used a rubber tipped rack handle to test mine. Hope you were successful.

 

That turned out to be a bad connector, an ez fix. Now getting the interior trim panels back in place is my newest dilemma. The yellow plastic trim fasteners seem hard to find. None of the auto parts stores locally carry them it seems. They carry something close, but not , the collapsible, hollow stem short yellow ones.

 

Can you provide the contact information for the fellow who rebuilds the brake light strips?

 

Here is the URL: Cadillac LED Replacement 3rd Brake Light, Third Brakelights For Escalade, ESV, Escalade EXT, Avalanche LTZ, Chrysler Crossfire, 2010-2011 SAAB 9-5, 2015-2019 GMC Yukon, Yukon Denali Tail Light Repair