I don't know if the '99s are the same as my '92, but on mine there is a fabric covered rubber molding that secures the opening of the headliner around the sunroof to the metal opening in the roof. There is a clip at the rear center of the opening that hides the seam in the molding. The molding just pulls off, releasing the headliner board from the metal edge. When you put it back, you need to be careful not to roll the fabric off the board as you re-install the molding. Also, the molding tends to shrink over time, so don't push it in too far at the corners or you won't have enough to cover the gap at the rear with the clip.
I don't recall if that will give you enough room to get to the drain fitting in the corner.. If not, you may have to undo your visor clips and pivots to let the front of the board down enough
Mine was leaking right where the plastic tube meets the rubber corner piece. I re-sealed mine with black RTV silicone sealant. Dry now.
Hope this helps