Do you have a sunroof? THe quickest way I found to put my back on is to undo the weatherstrip lining the inside of the sunroof. The seam is on the driver side. Then you can slip your hand between the roof and the liner and hold onto that loose black plastic piece and clip the light in..
Before I figured that out, (it fell out a bunch of times) I got a wire coat hanger and undid it. Then I fed the hanger around the back piece toward the edge of the hole and looped it around so I could hang onto both ends. Then pulling down on the black piece, I pushed up my dome light and when I heard it click into place, I very carefully pulled the coat hanger out.
- that light is SOO F'n Annoying.
-kou5oku