3rd Taillight - How to get to it to check connection

[jetrocker]

The top of the tailgate 3rd lights (brake lights) are sketchy in terms of coming on. Sometimes they do, sometimes they're dim, sometimes they don't go on at all. I suspect a loose or corroded connection. I don's however see how to get to the lighting unit and wiring connection. Can someone give me the instructions? 2010 Model. Thanks, Mark