Hi
It depends on what is the matter with it. My roof completely failed to open, I discovered that the cables had worn. Listen to this carefully it is not a mistake, Fiat only supply the complete roof cassette, they do not supply the cables on their own! The last time I asked it was £1800 plus vat plus fitting, and your right, to remove the roof the headlining has to come off.
I decided to leave it alone, I did find a place in Sutton-in-Ashfield that would make up the cables if I could get them out.
But I had a bit of luck, I bought a complete new roof in the box for £100 ! off ebay. It was very time consuming to replace though, but for £100 I wasn't complaining.
I have just bought the complete FIAT workshop manuals (ring binder version) from ebay and you can remove the glass by removing the stops etc. This way the cables can be replaced.
The roof does tend to stick if not used for a while
The only 'workings' you can get to are under the panel where the rocker switch is, all the rest are on the outside, so don't take the lining out expecting to find anything.
More information as to what exactly is happening would help.
Cheers