If the car didn't ship with the HK system, then the two holes which the HK subs fire through are pre punched in the rear deck sheet steel.
From the trunk, look under the rear deck, towards the centre, there should be two circles which are pre stamped. Just a simple tap with a mallet and they can be knocked out, leaving two holes thorugh the deck which currently are unsed.
This is all assuming of course that you don't have the HK system - if you do, then as Johnhab has suggested, just remove the HK subs and eiother fabricate a baffle for some IB loaded drivers or that alone, will allow more air flow from trunk to interior. You could always go for a ported enclosure and vent the box with a JL flew port through one of the pre provided rear deck cutouts.....