BMW Repair Questions? Ask a Mechanic for Answers ASAP
If you have a registration I will see if there is any wiring diagram available.
Has the vehicle got single zone or dual zone heating system?
Please find attached a wiring diagram for your Heater / AC System
Thee is a link within the system to the Audio Unit. I suspect you will find it is a module issue that is causing this problem however the first thing to do would be continuity checks on the wiring and recheck that there is power and earth to the units.
There is no dedicated wiring diagram available on Autodata for the Audio Unit.
I would think your best option for diagnosis would be a BMW dealer or a BMW specialist.