amp wiring to factory h.u.?

Woops...sry about that

I have a new Body style GMC Sierra truck with standard non-bose H.U. and Onstar. I am going to install a amp to my stock head unit. I already upgraded the front/rear speakers. Here is my question… My amp has high level inputs. Do I just intercept the speaker wires behind the H.U. ,go to the amp(high level input) Then from the amp to the other end of the cut speaker wire? I know it would be better to run new speaker wire but at what point does Onstar tie into the system?

Thanks in advance for your time

Yes that is how you do it i Dont know about the onstar thing but if your only cutting the the audio cables which is what you want to do im guessing it shouldnt effect nothing just get the polarity right