Novice needs help

Hey guys... Im trying to add a sub and amp to my 2003 suburban(w/bose). I have a few questions and was hoping someone could help. 1. My amp has "speaker level inputs." This means I can tap into a speaker and get a signal without getting into the dash and running RCA's back to the amp, right???? 2. If #1 is correct, is there a way to verify which color wire is + or -? There are a brown and a yellow. I think from what I have read, brown is +, but not sure. 3. Anyone have any ideas for my turn on wire? There is a wad of wires running through the back of the truck. Is it possible I could turn the key to "on" and see which one has power, and tap into that? Hope thats not too much....thanks in advance for any help

https://www.the12volt.com/installbay/forum_posts.asp?tid=27878&PN=1&fid=2&PR=3 go there first to  find your wiringHead Unit:Alpine CDA 9835

6.5 MB Quart Pce 210

Amp:JBL 80.4

Amp:JBL 1200.1

Sub:Adire Brahma 12 MKII