Here are the links to 2 outstanding build logs
https://www.diymobileaudio.com/threads/2021-gmc-sierra-helix-audiofrog-gb-jl-resonix-the-revolving-door.444799/
https://www.diymobileaudio.com/threads/alipors-2019-chevy-silverado-high-country-semi-stealth-system.416775/#post-5690565
You can run wire from under hood just behind battery there is a rubber boot you can go thru
i did this in mine
https://www.diymobileaudio.com/threads/2021-silverado-rst-build-log.442362/#post-5979033
as far as the back wall for mounting amps I am unsure. I bought a HDPE from sounds good stereo online and will be installing soon.
there are a couple of builds on this site as well as DIYMA
There are a few more build logs as well including one on a non Bose that was informative
You should also check these out as well on DIYMA
https://www.diymobileaudio.com/threads/alipors-2019-chevy-silverado-high-country-semi-stealth-system.416775/#post-5690565
This one is mine. Haven’t gotten very far yet…
https://www.diymobileaudio.com/threads/2021-silverado-rst-build-log.442362/#post-5989760
Smack dab in the middle of the back wall along with the inverter
have to remove the back seat to get to it
there are a few tutorials on YouTube on how to do it
Do you still have “chimes” and alerts?
These along with turn signals are played through dash speakers
You may want to check to see if the loop harness utilizes the dash speakers as a source of input. Do you have the Bose setup?