2018 Silverado aftermarket audio wiring help

[Anthony Ventura]

Hi -
I'm not new to car audio and have installed my own system in my old 2008 Impala. I had an aftermarket head unit with three rca outputs. I have a crescendo bass clef bc2000d for my two kicker cvx 4ohm, and a pioneer gm-a4704 to power my mid's and highs which are two 2.75",  2 6/9, and two 6.5", all Kicker. 
I have seen people splice into the wires and run to their LOC. I have never used an LOC. I am going to buy an audio control lc7i, and was planning on buying a wiring harness. Would I have to run the wires from the harness to the back of the truck into the LOC? Could I tap into the wires going to the speakers instead? Would a harness even be worth it? I'm confused on the wiring.
Another issue is the box. I've having a hard time finding a ported box to fit the subs. The have a 5.75" mounting depth. I know I need to buy a seat lift. I found a few people that make them online but they aren't very responsive. One is Subthump.com and Soundoff audio. He want's $400 which is crazy. If anyone can help me out with these issues I would greatly appreciate it! Thanks in advance!

[ftwhite]

Are you using an aftermarket head unit, or are you trying to tap into the factory head unit?

I guess I assume factory, since if it was aftermarket, you just run RCAs to the new amp. However, with factory, it depends if you have the Bose system or not. In codes, that's the "IO6" or "IO5" system. If that's what you're dealing with than you have two paths: 1) use an LOC, and tap into the speaker level outputs to feed your sub; 2) spend some money to by a unit that taps into the MOST-50 signal that GM uses. This later one gets you a flat RCA level output that contains AND attenuates the environment sounds produced by the car and OnStar. If you go LOC, I believe these sounds get amplified based on the music volume. 

Depending on the system, you might have 4 door speakers (front and rear) and two dash speakers, plus a sub. So up to six drivers and 6 channels (R, L, C, S). 

I went with a NavTV M650-GM. Expensive, but it gets me a unprocessed, flat RCA level output that I can feed and process the way I want to.