Hmmm. . . that's about the same route I plan to take. . . mounting the amps on a re-enforcing sheet of plywood (or something lighter/stronger) behind the back seat. And the sub off to the side. . . probably a custom fiberglass enclosure. . . lightweight, airtight, and easily moved.
My only concern is for mounting amps a capacitor(s) on the backseat. . . will we lose space for rear upper strut brace and corresponding V-brace? I really want to keep enough space for that. I'm hoping once I pick up the STi that I can measure things out and see that the stuff will fit nicely between the seatback and the bars (once installed).
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