If the voltage is low before you start the car the ECUs may not fire up correctly, and they don't generally recover even when the voltage comes up. That said at that mileage it's probably due a rebuild as well.
I have at least 3 speakers not working in my L322 but I can't be bothered taking the door cards off and scoping the inputs to see if it's the amp or not. :)
I'd suggest trying the phone module as that's been messed with. You should be able to see a light in the fibre and a light in the module where the fibre goes. The fibre with the light goes in the module connection without the light.