To be exact, that mount is NOT a BMW Nav II or III mount. It is a standard Garmin 2810 or equivalent. BMW Navigator mount has the extra four buttons on the left side of the mount.
Therefore, since the mount is Garmin and not BMW (which of course is also made by Garmin) I presume that the cable is also standard Garmin.
This leads to the doubt that the cable with all the pins is only using two pins +12V and ground, therefore it is only supplying power to the GPS unit, no sound or anything else.
Original BMW Nav II / III harness has a connection to BMW Intercom for receiving the GPS voice commands. For non-BMW intercoms there is a separate 2,5 mm female jack that supplies GPS sound to Autocom, Baehr etc. However, the GPS units at that time were not designed for music listening and thus there was no stereo output from GPS.
If you find it hard to find an older Garmin / BMW GPS you can use the base mount for a new Garmin Zumo 660 (or BMW nav IV) , for instance.
You just unscrew the four allen screws and install a Zumo mount instead.
However, you need to replace the existing power cable with the Zumo harness. That has all the outputs (and inputs) for connecting it with your bike intercom as well as with your cell phone if you wish.
Regards