BTDT.

Two things:

First, don't spend any money yet. The "gearbox" (for lack of a better term) is secured by two screws. Those screws attach into metal tabs that, themselves, are held to the roof frame by two screws. These four screws loosen after time, and the "gearbox" sags. When this happens, it doesn't properly engage with the cables. This misengagement causes the "clicking" and skipping and lack of closure.

Tighten all 4 screws. If they don't tighten, then one or more may be stripped. On my '83 GL with this problem, I retapped all four screw holes for 3/16" flathead screws (from the hardware store) and socked it down solid. The sunroof opened 'n closed flawlessly after that.

Second, the "gearbox" has a built-in turns counter. It will only wind or unwind about 14 turns in air. So, if you simply reinstall it, you might crank it to its internal stop, but the sunroof is still open.

The solution is to remove the gearbox, unwind it a few turns, and then reinstall it.
--
Don Foster (near Cape Cod, MA)