I've just been messing with my sunroof as it developed an annoying rattle. Here's what I've learned...
There are some runners you can see when it's open, so they're easy to clean and lubricate. But there is also another runner behind it, which is concealed by the plastic trim down the edges of the roof. I removed the roof rails, then used a plastic bike tyre lever to gently prise the plastic trim off. It's just held on by friction clips. This is for a car with roof rails - it may have other fixings without the rails, as the roof rails do clamp it down.
After getting the trim off, the runner is visible. Give it a good clean and a healthy squirt of white grease. The same for the front runner. Then remove each of the two torx screws that hold the sunroof up when it tilts (one at a time!). This is where my rattle was. Clean both halves of the bracket, douse in white grease and screw back together.
To replace the trim, remove all the little clips left behind in the car roof and clip them onto the tabs on the trim. There are silver and black ones, for each of the two sized tabs - I can't remember which is which, but it should be clear. Mine was full of grit and filth, so I used the Hoover nozzle and a paintbrush to clear it all out, and for the runners and other areas too.
I also removed the double-sided velcro pads that had been used by PERRYS of PORTSMOUTH to attach the trim after they obviously couldn't work out how to get the clips out of the holes and back onto the tabs. They had also bent some of the clips by leaving them in the holes and stuffing the trim back on, presumably using a hammer. I will never ever trust that cowboy main Fiat dealer for any work on any car I own.
After doing this, mine's working great. It almost worked before, but not very well. Mine had a new mechanism fitted by Perrys of Portsmouth under warranty about 3 years ago, but it made little difference. If indeed they did replace it - I now have no faith in their work. Grease is the answer.
I also found a liberal smothering of trim polish helps the sunroof slide past the rubber seal as the brackets pop up past it before sliding back.
If you get stuck then there's a manual winder within the rear interior light. The handle for it will either be clipped inside the light or in the glovebox.