mine is a good bit older than yours, and used a completely different marker light fixture, but I'll bet they are built in a similar way.
in my overcab bunk there's a small box built spanning all the way across. I tore into it t gain access inspecting for a rot/leak problem. It was clearly built from outside so it was kindof a bear to get into without tearing it up too badly. Once inside it was just an empty space extending up between the finish ceiling board and the outer nose cone skin. It's been a while, but I believe the wiring came from in the roof, driver's side. Not reachable form where I opened it up.
Glancing at your picture, I'm guessing you are hoping to get at the wiring for the missing fixture. Is that right?
Just guessing, but I'll bet that the easier approach might be to pull out the fixture next to it, then you'll probably be able to pull out some of the wire or fish around in there to get at it, then use a fish tape to reach it back over to the hole. The lights are probably just connected in line, one to the next to the next.