Agree with the other.. try the joint first, as mine have come off there several times... Fingers crossed seems to be OK since last fix...
As for how to access the High Level Brake Light, you need to be flexible....
Open the tailgate. Inside the tailgate (at the top) you will see 4 nuts. Remove them one at a time. They will then expose 4 "threads". These threads are attached to the rear spoiler, which the high level brake light is in. Now the flexible bit.. You need to lower the tailgate, so you can gently lever out the rear spoiler. Do this by gently tugging one side, then the other, back to the other, etc, as you want it to come out at straight as possible, to avoid scratching the body. When free, there is a cable connection and a pipe (rear washer). So be careful disconnecting these, so you know how they go back on.
If you can get someone to help, ie. hold pieces it helps.
BUT I bet the pipe has come loose in the rear quarter as stated by others, as I went through the exercise you are discussing (high level brake light) first time it happened to me.