This works for me, thank you!!! I have an AVH-4100 / Note 8 / Gear S3. In order to get it to work I had to wait until my phone is connected to my head unit with bluetooth before plugging in the USB cable, otherwise it disconnects immediately upon plugin of the cable.
Tests performed:
1. Turned on the car and plugged in the USB cable prior to the bluetooth connection being established to my AVH-4100. This resulted in the watch disconnecting as soon as the Note 8 bluetooth connection was made to the AVH-4100.
2. Turned on the car and waited for Note 8 to connect to bluetooth on the AVH-4100 then connected the USB cable. This kept my watch connection. I then turned off the car without disconnecting the cable first and my watch connection was lost, as expected.
3. Turned on the car and waited for Note 8 to connect to bluetooth on my stereo then connected the USB cable. This kept my watch connection. Disconnected the USB cable prior to turning off the car. This also kept my watch connection. Success!!! Now I just need to remember to do this.