Here is a link to the people that many here have used to repair the light. I would check the wires and connectors first though.

Cadillac LED Replacement 3rd Brake Light, High Mounted Stop Lights, Third Brakelights For Escalade, Escalade EXT, LTZ, ESV, XLR, Seville, SLS, STS, Chrysler Crossfire, Lexus SC 300 400, Hummer H3