If I remember correctly the plastic part (cable dog?) can be unscrewed from the cable with a philips head. I would have thought it would be a fairly cheap part form the dealer (if its in stock). It is fixed in the runner so the PO must have snapped it off somehow with the cable. Once you get the plastic part back in the runner you will need a narrow philips to re-attach it to the cable to it with the screw. If the cable is still in its guide tube then it should still raise the roof when moved by hand if it doesn't then probably mean there is a prob else where - up top in the gearbox at the roof, stripped cop etc (be prepared for that as it would explain why the cable would be snapped in the first place). see the garage manual on sunroof O, T & R at
www.clarks-garage.com