Sub question

RonRizz

You will not need a Capacitor, nor will you need to upgrade your alternator. I am currently running 4 amps (for years now) and have no issues whatsoever. 2 are 300 watt, one is 400 watt, and one is 1000 watt.
If you value your hatch space, go with a sealed box.
a single 12" sub will have more than enough punch for anyone in the Z. Average box volumes for a 12" Sub are around 1 cu ft, although there are a few shallow mounts on the market that will work in 1/2 cu ft.

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P99rs Head Unit--- Amps- 2 Pioneer Prs800d --- Zapco ST1000MX---Zapco ST-4XSQ---- SI BM mk5 Subs---SI TM65v2 Midbass--- SI M3 mids--- SI M25 xbl2 tweeters ----- 2016 MECA street class 3rd place at world finals