try and find the earliest h/u that plays mp3 at the same output voltage and go from there.old alpines where mostly 2v and up to a couple years ago they still had 2v,hell they still may have 2v outs
the strongest points of your body are seatbelt and brake pedal mounting location to which both are great places for a ground..just make sure the wire terminal is on top of the seatbelt mount
you need to find your vehicles freq,you can do this with a radio shack spl meter at low volume,just do a sweep from say 30-60 hertz and see where it peaks,thatll be you cars resonate freq...meter is like 56.00 bux for the digital,and worth it
you should have everything mapped out for what you want as apposed to building a sub system around an amp...sell the sony to some guy starting out,on craigslist then save and
research what you need to fit your goals,if not youll end up spending more money than needed,allthough sometimes thats the fun of it