I've been collecting parts to upgrade the base audio in my 2011 X5 for a while now and came across something that could simplify installs greatly. A review for it is below:
https://www.educarmag.com/reviews/pr...kicker-keyloc/
This device takes in the speaker level audio from the amp/headunit, converts it to line level, and automatically performs digital signal processing to remove any stock audio signal processing. There are test tracks to do and other function, but just this alone seems like a real boon. Each 2 channel unit costs roughly 130 USD.