I am about to upgrade the sound system in my 2007 350z. I will be doing the following...

1. KEEP the stock head unit (i know...i know...but for numerous reasons -- this is my only option at the moment)
2. Replace the front components with Diamond Audio D363.5
3. Use the front speaker wire inputs as HIGH level inputs to a 5 channel amplifier (SoundStream RUB5.800 -- I jumped the gun on the amp before I realized that changing out the rears will not play as much of an impact as far as a sound stage is concerned...also I imagine it will make for a clearer sound - because right now the sound bounces off the headrest to the rear then comes back to the front adding a time delay between the front spkrs and rear spkrs - i could be wrong though).
4. Use the amplifier to power the front components and a 10" Infinity 1060w in the trunk.

My first question is fairly simple. HOW DO I WIRE THAT? lol...I mean how do I identify the wires which are currently coming out of the stock head unit and going into the front components (before the signal is filtered for the tweets and woofer -- if the signal is filtered at all). If I can identify the wires (positive / negative) before they are filtered for the current tweets and woofers - I can feed these inputs into my amplifier as high level inputs, correct?

My second question is how can I feed my speaker wires from the amplifier back to the front speakers? I mean what path do I take (as well as how do I take it) when I meet the junction of the inside of the car and the door itsel? Thanks guys.