The rear window, with it's surrounding mohair section, is handily seperate to the rest of the roof. It's attached to the rest by:
- a zip along the top (accessible from inside)
- pop-buttons on elastic in the two top corners (inside again)
- velcro down the sides (easy from outside)
- apparently bolts, but in my case it was rivets, along the bottom edge - these are best accessed by undoing the other fastenings first (as they're all easy to undo and redo) and then releasing the hood (as if lowering it) so you can see the underneath of the bar
It won't take 5 minutes to undo the top and sides if you want to investigate next time it's light and dry.
Replacement sections are available as an Alfa part (I gather in the range of £300-£400) or off eBay - I bought one last year for £140 and it's fine as far as I can tell, still using it.
You can fit it yourself, although I made the mistake of fitting mine in cold weather (it was, in fairness, January!) which has left it slightly rippled. It can of course be re-done or adjusted, fairly easily if you have bolts, so if you're feeling handy you might earn some points from your other half!