So you want to put 6" subs in the area below the side back windows? I tried this a long time ago, never do it again. The 6" subs (actually midbass) will not contribute much back there - especially if you will have 2 12"'s behind the back seat. Kind of like trying to talk during a concert (what did you say???) . If anything, put them up front. Focus on building the best, loudest, cleanest front stage you can. Most of the real good systems have minimal if any rear speakers.