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