BMW E46/4 (sedan) HK 2-channel Subwoofer Amplifier
TL431 is a adjustable shunt reference most likely used to set up a virtual ground of around 6 Volts since normally you would want + and - voltage for the audio circuits.
The amp is from NXP, and even though it's advertized as 40 Watts x 2, it's probably more realistically like 20 Watts per channel into 3 Ohms.
|X18774||Description||Wire Size and Color|
|5||Fuse 41 - 30A||2.5 RT/GN|
|7||TAA, Speedometer signal||0.35 SW/GN|