I am trying to put a 12" sub and amp in the trunk of my 2000 i30. I have the power wire run as well as the remote turn on (am planning on hooking it into a fuse that is only on when the key is turned). I am wanting to tap into the wires i see in the trunk on the back of the Bose sub to hook up a speaker level to RCA converter box but needs some help knowing which two wires i should use. In the future i am planning to replace the stock head unit and will run the RCAs from that but for now i would just like to get the signal from the easily accessible wires in the trunk. Thank you

Just tap into the wires directly running into the speakers.

How well does it work? The Bose stereo seems to "hold back" the bass output when you crank up the volume, as if it's some sort of way to protect it from overload.

I was hoping to tap into the sub, there is a whole harness of wires coming from the amp to the sub. I want to find out which two of those I can use to hook up the adapter if possible. They are the easiest to get to and I am hoping that way I will have more control of the bass coming out of the sub I am adding using the bass control on the head unit.

There's a whole harness coming from the amp to the sub because the sub has its own built in amp. You shouldn't need a converter because it receives line levels from the main amp. Check the wiring diagram in the FSM.
http://www.nicoclub.com/FSM/i30/2001/el.pdf

GreAt link. I am going to check tomorrow and make sure I can figure out which wires are 43 & 44 in the diagram and go from there. Thank you for that