In my opinion ...
Ditch the jack storage compartment and custom build you a sealed dual 10" enclosure ... You have to have 4 8s to sound as good as 2 10s when built properly ...
1 12 never has sounded good under the rear seat in my opinion (not enough tuning capabilities) ... not enough air space for 2 12s ...
a pair of 10s in a correct built enclosure will suprise you ...
I'm in Sound Quality ... I'm not a Basshead ... Well, atleast not in the truck I have now ... My 91 Chevy had the wall cut with 4 15s in the bed at one time ...
Don't go cheap on the amp / battery power / cabling either ...
Remember ... subwoofer setups are ONLY as good as the battery power / amplifier power can deliver ...