It was exactly that, the CCM.
No water on or near it, very close inspection showed no dry joints or hot components, but as soon as I changed it for a 25 quid secondhand unit everything jumped into line and worked.
I'd previously changed a whole host of things ... door locks, rear wiper motor, rear boot lock, checked a fair bit of wiring and a load of stuff I've forgotten, but the CCM solved the problem.
Given how difficult some of the wiring is to get at, if you can find a CCM with a matching part number at the right price, I'd try changing that first. On the Golf you have to code in the number of doors and electric windows, but Passats are probably all four door anyway and most likely all with electric windows.
Coding is quite easy with VCDS I've got a full version if you need to do it.
Thinking about it VCDS will test the interior lights along with loads of other stuff ... all the instruments, the display, buzzer, ABS, I think fuel pump and loads more items I've not yet played with. If you've not got a copy and want to pop up for a play you're very welcome ... you'll probably understand it all way better than I do!