2-3 10s ,1 15, If you can fit 2 12s in that space and have good out put(depends on the subs)good luck tuned to 39-35hz should be very nice improvement . the box will need to be built on point .
'a couple of 12s' is the problem. Which 12s would be louder than my Fi that meets my space requirements. Idc about sealed or ported if it all sounds right. Back seats also do not go down
well, I cannot visualize what the back of your seats look like, or if the seats are fabric, leather, etc. But an old school trick is to cut the foam out of the seat and use seat belt straps across the void tied to the metal seat framework, like this:
I am just tossing ideas out there for you to mull over...like maybe 2 of those Tantric 10's or some SPL/PSI/IAD subs sealed from the trunk with the subs/port facing to the back of the rear seat..