Issue with Rotten ceiling (duetto)

Good afternoon, 

I think I might need some help.  I have a 2001 Duetto based on a MWB Transit just before xmas I noticed that habitat lights in the back started to play up all the lights on the left (looking forward) aren't working...  I replaced Bulbs no help.   Today I went out and looked again that it think that I have established that wiring in starting at the back right and working round to front right then across...  (the working bulbs all have 10 volts, the others have less that 1) ... so 1st question:

Does anyone know where the wiring is running?

Next and more important as I was there I looked at the bathroom light and it transpired that the ceiling board is no longer there and rotten away...   so the question has anyone replaced the ceiling boards or know how the boards are set up?

Thanks in advance

Typically the ceilings are simply thin hardboard panels covered in fabric and screwed in place, so there isn't really anything complicated about them.

But I guess the more important question is how the ceiling panels came to be rotten - realistically this can only happen due to water ingress - do you know how, and has that problem been fixed? That obviously needs to be resolved before you can do any replacement.

Thanks, this is going to be the 1st time taking a van apart and cant see anything immediately obvious for taking ceiling out... or if its in sections.   As I only became aware yesterday and the wood that is there still feels damp I am assuming the issue is there...  though I did just recently get some small patches of rust dealt with

Perhaps another Duetto owner (or ex-owner) will remember the layout, and likely source of leaks at that height. The only ones that I can think of offhand are heater exhaust, roof lights and (if fitted) TV aerial. The roof side windows are a known leakage point but even with capillary action I suspect they are too low.

But no point in putting any great effort into replacing panels or supporting woodwork until you've found the source, IMO.

I had the roof windows resealed last year and TV aerial plugged (don't have TV aerial and use internet).   The radio aerial was leaking 2 years ago but that was sealed and no leak in front of van (it apears) bathroom seem worst point