Sagging sunroofs are generally caused by the separation of the headliner fabric from the headliner foam. Once this happens the fabric will droop down farther and farther as its own weight (and wind) cause it to pull farther away.

As long as the backing board is in reasonably decent shape you can just scrape it clean and reapply a new piece of headliner (you can buy it from any fabric store... $75 or so for enough for a wagon).

Make SURE you scrape that backing board clean of the old junk first. If you don't the new headliner won't have a good surface to adhere to and will just pull away in teh near future. Also, use the glue (get good fabric glue from teh fabric store, NOT a hardware store.. ask the guys there for the one they use for headliners, probably 3M or something) LIBERALLY and do the headliner in sections (back to front or front to back). The glue dries at a good clip, and you will be too rushed if you try to do it all at once.

Last note: remember to leave 6 inches of excess fabric around the sunroof opening... everyone always forgets this and cuts it flush, screwing themselves by leaving to liuttle fabric to hold to teh sunroof lip.

good luck,
rt