I recently completed this mod on my 2021 Tahoe Premier. I did the Alpine sub with the boss audio Amp to keep the cost down. I got the box off of Ebay as a lot of the online parts sites did not have it in stock and didn't know when they would get it because of the strike. I ended up paying about $180 shipped.
I will try and answer your questions.
-LC2i - I originally skipped doing the LC2i, but ended up adding it to the mix. The bass roll-off or attenuation is pretty significant. It should be here tomorrow and I plan to hook it up this weekend.
- Speaker wire to tap - Lancecc1 posted a youtube video on page 5 of this thread that shows tapping the speaker wires in a silverado. I did exactly as they did in the video
- Hardware - There are two existing bolts on the floor behind the side panel you used to attach the box at the bottom. Then I got a few washers and nuts from Home Depot to help secure the box on the top. That was it.
- I don't know about using a power lead in the rear. I did mine from the battery as in the video. It was pretty easy to pass through the firewall and then I used fish tape to run the wires from the front to the back.
- I used the existing Grommet to run the wires in to the box. You can pop it out pretty easily and then do what you need to do with it before popping it back in. I used a drill to make the holes bigger to pass the speaker wires through.
- Yes, I left the front console sub alone.
The youtube video showed how they tapped the front fuse for the remote wire. In my Tahoe the fuse they used provides a constant 12V, which would make the amp never turn off and drain the battery over time. I ended up using a Micro3 fuse tap on fuse F26 for on/off functionality
The only other issue had was figuring out how to detach the USB wire holder from the side panel. I was afraid to break it, and it took an embarrassingly long time to figure it out, but I did.
Thanks again to everyone who shared their knowledge on doing this mod. I am not super handy and have only done one other car audio install back in 2018. So, if I can do it, you can too.