here's a link to some pics how to do it.
https://mazda3revolution.com/forums/2014-2017-mazda-3-skyactiv-audio-electronics/160746-using-alpine-ktp-445u-2014-mazda3-tau.html#/topics/160746
1) on the pac unit. cut the wires going to the speakers.
2) run your speaker wires from the amp to the...
The "tether" isn't complicated. just get needle nose pliers and turn the connected piece til it lines up with the hole and pops out.
its a pretty robust trim clip. you wont break it.
I found it was easiest to remove that white clip and slide it onto the door panel before putting the panel back on.
panel slides over white clip here.
use a flathead to pop out. Held in by tabs and pops into place when u put the door back on