Delco-Bose CDM Bluetooth mod

My goal is to keep the car as original as possible and the original Delco-Bose Gold radio system sounds great in my opinion. So a radio swap was off the table.
Some internet research revealed a possible modifications to the head unit but there seems to be some drawbacks. I listen to the radio quite a bit, but would also like to have the option of receiving a phone call through the system. However, the radio receiver is located in the CDM (also called Radio Receiver Box) of the system and not in the head unit. So adding Bluetooth to the head unit would give me the desired functionality.

The first step was to find a Bluetooth receiver that also provides an AUX input.
I found two boards on Amazon, the Comimark LN-BT02 and the ​DROK 200367 Blue-Tooth 

Testing of both boards showed an interesting behavior.
The DROK board switches from AUX input to Bluetooth audio as soon as Bluetooth connection is established, while the LN-BT02 only switches to Bluetooth audio when an audio signal is present vie Bluethooth.

This is exact what is was looking for. With the LN-BT02 i can listen to the radio and it automatically switches to Bluetooth audio if a signal is present, e.g. a notification, or a phone call. Another benefit of the LN-BT02 is that it also provides an microphone input and therefore can be turned in a full handsfree setup with an external microphone.

Delco-Bose CDM modification

This modification applies to the CDM #16178351.

Open the top cover of the CDM and locate the radio audio traces:

Connect the Auc and Output wires of the Bluetooth modue as shwon below: