drop the headliner. get a 1" X 4" piece of lumber about 6" long and a BFH, start where the roof should be curved about 2-3" away from the center of the dent.
once you are finished fill the gap between the roof and the glass mldg with calk to prevent water leaks.

NOS HIT NOTE***don't miss the lumber with the hammer or it will go thru the glass. DAMHIK