What kind is it? If it's off ebay, it's the ones you have to usually BOLT right though the hatch, like drill and make holes.
The 'brand' name ones are mounted with two sided tape. Personally, if you plan to always keep it, get the body shop to just glue it to the car. It'll stay on where it should longer. I had some issues with the tape pulling away slightly (after 5 feet of snow and -50F up here in Canada) so that's probably why. They glued it on, no problems.
I'd try to stay away from actually drilling though the hatch.
My Wings West mid wing I mounted with 2 screws on each end of the center section plus tape, and the smaller end sections are just tape. Just make sure that if you drill the hatch to use screws that you paint the edges of the screw holes real good before you mount the wing to seal them up from rusting. Also make sure to leave a section about an inch long in the center of the wing without tape so that rain water can drainthrough it otherwise it will puddle up on your wing.