I have a TV tuner with a £10 bluetooth gateway plugged into the AUX IN - it works fine but when the engine is running there is the ground loop feedback whine which is pretty much unavoidable in this setup. Eliminating that with an isolating transformer reduced the quality of the audio making it have a dull tone. It then packed up after just a few weeks before I had chance to remove it.
I have seen you can now purchase MOST BUS MMI 2G (and 3G) bluetooth gateways from ebay for £123 and it integrates with the AUDI MMI and steering wheel controls for next track etc... It is made by MR12VOLT
https://www.ebay.co.uk/itm/MMI-2G-Bl...F/112554298106
It has DIP switches where you can configure it for different cars such as PORSCHE and VOLVO - here you can also disable the handsfree phone function. I am attracted because it uses the fibre optic MOST bus and so the audio should be highest quality possible and also due to the integration. It will appear under CD as CD9 or CD13 depending if you currently have 1 or 2 CD changers. It would install next to the existing CD changer and piggy back of it's connections. The seller has sold 113 but I am unable to find any reviews.