If his brake lights work and just not the 3rd brake light, maybe check the 3rd brake light connectors to see if they are getting power? Volt meter with the door open to test since they only come on when brake, door or cargo lights are on...I haven't even noticed if cargo light turns on if you hold this light switch for interior to dim or turn brightness up ever.
Sorry for the long post but here it goes... My truck has had the famous "Hill descent control fault" flash a few times while driving before. It usually happened in wet weather and always went away. For the past week it has been raining pretty hard here, so the fault has really been an issue...
Im sure there is on our forum with this information as well, I just did a Google search. I had no about how the HD and all that work with the brake lights and such.