Audio gurus please school me.
Mods please move if wrong forums.

I know about as much about car audio as I do about quantum physics. I know it exists and only have a rudimentary understanding of how it works.

So I have this collection of parts and need some help sorting it out. The Mustang currently has a system in it (came with it), the Frontier does not have a system, and I need to clear the extra system parts off my parts shelf. So please direct me what I need and what I should swap (if anything). Goal is to make room on my storage shelf and not spend any money, LOL. I also rarely (never) listen to music in the Mustang.

Mustang:
Pioneer DEH-P3100UB head unit https://www.crutchfield.ca/S-wSy8M5I...g&awug=9000829
Kenwood KAC-729S amp. https://www.crutchfield.com/S-bkQJLC...-KAC-729S.html
Kenwood KFC-W3003 12” sub. https://www.crutchfield.com/S-wHCtkt...KFC-W3003.html
OEM (?) speakers.

Frontier:
OEM head unit (no pre-amp outs)
OEM rear speakers and dash tweeters
Infiniti Reference 6x9 front speakers. https://www.crutchfield.ca/S-7rD5SAA...ce-9633cf.html

On the Shelf:
Alpine MRP-F200 amp. https://www.crutchfield.com/S-eocoAl...-MRP-F200.html
Sony XS-L101P5 10” sub. https://www.crutchfield.com/S-xFO11m...XS-L101P5.html