Hey Mo If your alternator is charging fine then I would suspect a fault with Battery Monitoring Sensor. If you look at the negative battery terminal, there will be a connector with 2 pins. 1 signal and 1 positive. You will need to do wiring checks to make sure there isn’t an open circuit anywhere along the wiring. The positive travels from the positive battery terminal and go to the BMS (this also has a fuse in the circuit). The signal wire (Lin Bus) travels from the BMS to the Body Control Module in the passenger footwell. I would suspect the Lin Bus wire may be at fault as I have come across this before.

Hey Aaron, was it all working before the battery going flat? On the infotainment screen, does it show the volume level'? If the amp has been replaced then it may need programming to the vehicle which will include it being updated to the latest software level. Have you been able to get anyone to read the fault codes? If you have access to wiring diagrams I would suggest checking the powers and grounds to the amp before moving forward.

Was the car told it had a new battery? Might be worth trying that if you know anyone that has a machine capable. It would be worth confirming that the alternator is outputting correct voltage when running and stays at the correct voltage when consumers are turned on. If those things check out then it would be worth checking the Battery Monitoring Support wire which goes from the battery terminal to the Body Control Module. Last one I did only had one wire going to the BCM and I was able to run a temporary overlay to confirm it was the fault, could try that before carrying out a permanent repair if that is the cause.