It's been awhile since I 'fixed' mine, and it is now doing the same thing again, not popping the flap up all the way. There are 2 springs, one on each side, wound around the hinge-rod.

If you gently but firmly pull up on the flap just enough to slightly pull it past the highest position, they are visible, about an inch inside the outermost screws on the aluminum trim piece. But be careful, don't pull up too far and break the hinged pieces on the ends that the sunroof pushes down on to lower the flap when it closes.

They can be coaxed to work better by bending them to 'push' harder on the flap, as they get tired over time. The flap has to be completely removed to do this on the bench. Not a huge job, but as I said, the fix is temporary since the steel in the spring has lost its 'spring' (or maybe yours are broken).

__________________
1986 560SL
2002 Toyota Camry
1993 Lexus