You need to remove the cubby hole pocket. The one directly below the rear speaker housings. There are three screws in the front lower corner of the trunk, remove them and you can pull the pocket off. You will need to wiggle the pocket around a little to pull it out. If you have the seat off, it is easier to remove the pockets. But it's not necessary to remove the seat.
Behind that pocket, you will see the two mounting bolts for the antenna.