soundman's audio build and questions

[soundman98]

just picked up my '14 brz limited this weekend, but i've been trying to do research on the audio changes i'll need to make for a while..

questions: what do i need to know about the oem harness? the sticky kinda sucks and doesn't go into much detail... from my research, it seems i would require both the metra 70-1761, and 71-1761.
the 71-1761 appears to be the same gender as the connectors that are installed in the car to connect to the oem radio?

for starters, i just want to replace the oem radio with an aftermarket model.
later on i intend to gut the oem system and install my own gear/wire, using the oem harness only for basic deck power and/or accessory wire signalling.

also, where is the factory amp installed if i would have one?

future: current plans are full active 3-way system, it's unclear if i'll maintain the full active carpc-powered setup from my last car, or if i'll go over to something like the bitone, and run a carpc and head unit.. i'd really like easier fm/cd/aux integration again...

overall the high-freq speaker setup is already determined-- dayton audio 1-1/8" large format tweeters, peerless hds nomex 6.5" mids.. they served me well in the last car, and will fit this one as well.

the sub setup, i'd like to use one of the corner boxes with a single 8 or 10(if integrity concepts ever gets their box done... hint hint), i'm currently thinking of getting an 8" sundown sa-8 v2. heard many good things about them from audio geeks and bass addicts alike.

as far as amps, i already have an alpine pdx f4, and m6, so those should work perfectly for the current plan..

and lastly, for headunits, i'm kinda leaning towards the pioneer 80prs based on features, though i'm not crazy on the styling of the unit.. i'd much rather prefer the p99rs, but $2k for a headunit that i'll end up using occasionally doesn't seem worth it to me..

as always comments/suggestions are always welcome