amp sub on amp system

I was wondering if someone could please help me out.  I am installing an amplified subwoofer into a 2003 GMC Yukon Denali with premium digital sound.  The issue I am running into is that I see the 12V battery and ground on the wiring diagram but there is nothing as far as 12V acc is concerned.  I am planning on using the high level input connections.  If I use the high level inputs do I need to still connect the accessory lead?  The high level input wires should they be spliced into the front, rear, or subwoofer speakers?  Also, the system is already amplified, so will this cause an issue with me trying to splice the wires into the system?  Thank you.

What kind of amp are you running? Some amps are able to run without any remote lead. I would splice them to the rear speakers. The amplifier thats already in it, is it an amp for the speakers or is theyre a stock sub?MECP Certified

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A load of dynamat!

1982 Chevy Cavalier