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I can tell you how to do it, but that isn't the same as doing it. if you feel overwelmed take it to a professional pay him to do it.
As far as putting a Sub in the location behind the seat in a 350 Z, good luck! This was designed for a factory Rockford Fosgate system that had an enclosure behind the sheet metal. If your Z doesn't have the Rockford option, then the enclosure isn't there, just the opening. You'll have to build one to make it sound o.k. (build with suggested materials of MDF, Fiberglass, Polyester Resin, Polyfill, and Dynamat. These can get pretty expsensive before you start to build) IMO it never sounds that great doing this. you'd be better off in a separate enclosure, but I understand the limitations on space in that vehicle. Trying to get wires through the firewall is always a task. Doing it frequently in many different vehicles is the only thing that makes it easier.
The Z may require that youd disassemble son componets in the engine bay before you can even see or reach the wiring harness that go through the firewall. Then there might not be enough room for an extra 8gauge wire in that harness. So some people like to drill a separate hole just for the amplifier main power supply, Use a grommet in the hole before you insert the wire. Many installer have damaged vehicles by drilling in a poor location or not properly verifing
If you have doubts, ask, try, maybe search some more, or eventually give up and have a professional do it. Schedules around the holidays are very busy, so plan weeks ahead of time for having car stereo stuff installed. Most of all have fun, if it is too frustrating then something isn't right.