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.