You may have issues with your grounds.
Your radio is grounded up front, amp under the seat then an amp in back for subs... Not good.
Easiest thing to do is make a simple amp rach, thats what I did.
I cut up some 3/4" MDF to fit right under that formed square bar behind the seat. Take a look in the trunk you will see it.
I recessed some "T" mounting brackets into the mdf, covered it. Secured it to the top square bad with 4 self drilling/tapping sheet metal screws. It was pretty solid.