Dan8128

I'm looking to add an amp and subs to my stock bose system through the use of an LC2i line converter. I did as much research as i could, but i still have a couple questions,

1) For speaker inputs, is it better to tap into the rear speakers, or the stock bose sub? Is 18 gauge wire ok? i have seen people doing both and im unsure which is better
2)As for the 12v and ground, can i just run some 18 gauge wire from lc2i into the 12v and ground connections on my aftermarket amplifier? Or do i need to use a different power and ground source

Any help is appreciated! Thanks