But by the sounds of it...you nothing about car stereos and should have a professional set you up
blah blah blah. how else does anyone learn? i was told that about gear installs as well. did my own 4.10s and track lok. zero noise, zero vibration, and still 27 mpg highway!
start research.
if you want to keep the stock head unit, get your signal from the rear speakers.
you will then need a line out converter. (loc). cheap LOCs screw up the sound, don't cheap out. (or you can use an amp with speaker level inputs)
and either the LOC or the amp really needs a Low Pass setting.
so now you have signal. you need power.
pick your amp to match your budget and sound requirements. i like rockford fosgate fwiw.
then to power the amp, you need to run 4 gauge wire from the battery to the trunk, i used 18 feet of wire.
then find some subs with similar power requirements to the amp.
put the subs into a box that will fit into your trunk.
last, the turn on wire. that you can get from the power for the power antenna, since it comes up when the radio turns on.