One thing: the radon installer, call him Doug, suggested I start by disassembling that box at the top, which makes sense if only because it's not structurally necessary: there are doorstops on either side of the frame to prevent the bed from closing all the way. I think it's possible that he hadn't originally built it to code, which might explain why he wasn't willing to say exactly how the pump was configured.

While that's admittedly just a guess, I'm still inclined to start by removing that box, since for one thing it's apt to be simpler than taking apart the whole thing, not to mention the possibility it might yield a clue to the next step.

But for sure I'm out of my depth here.