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:

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Then select 37-Navigation:

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Then select Coding – 07:

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The Bluetooth module is in Byte 3, Bit 5 and is indeed disabled:

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Remove the checkbox to enable it:

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Click Exit and then Do It!

The Carkit is available under the Phone Button:

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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.