05 Corolla, where to tap amp input?

So I am playing with my new commuter cars' system. I have a hodgepodge of old stuff from years ago. MTX box for the trunk and decent Pioneer and Kenwood amps. Replacing the factory fronts with my old MB Quart separates. Not sure if I'll use my ancient Epicenter:)

I have done several systems years ago, but they were total gut and custom fab jobs. I just want to tap into whats there now and get a little more oomph.

Whats the best way to tap into the stock head unit? Should I use a speaker to line level converter? Should I connect the sub amp to the rear speakers so I can fade them out if I want? I want to be able to balance the system, though I guess I could use the amps gain.

Whats the common scenario for a small upgrade like this.

Forgot all the stuff about car audio I knew 15 years ago...

Thanks for any advice,

Chris

Thanks no use!

I think the amp has high level inputs. Should I NOT use those and get a dedicated loc?

I knew about setting and leaving the gain, and Thanks! for re-affirming what I thought about hooking to the rear speakers.