another day, another go. Managed to suss it this time.

<Ignition>
Holding down Select for 8 seconds brings up the Radio Service Menu.

<Ignition>
Press Info.
When TP and RDS show on the menu, pressing Info again will bring up the TP Options.

After adjustments etc, BT volume also sounds right again.
Now to tinker with the Map disc a little more. Did the OS9000 on VMware ever surface?