The Volkswagen RNS 315 Navigation has a builtin Bluetooth carkit which is disabled by default. I am not sure why, I presume this is done because VW also sells carkits that integrate into the MFD.
The builtin carkit can be enabled using VCDS like this:
First go to Select Control Module:
Then select 37-Navigation:
Then select Coding – 07:
The Bluetooth module is in Byte 3, Bit 5 and is indeed disabled:
Remove the checkbox to enable it:
Click Exit and then Do It!
The Carkit is available under the Phone Button:
Multifunction Steering Wheel and Streaming music over Bluetooth works. There is no integration into the MFD/FIS.
Note that, if not already present, you need to install a microphone.