In my 2007 C30 I have AUX port but no bluetooth. I was looking at handsfree kits but I'm not a big fan of 3rd party displays, buttons... And factory bluetooth retrofit is too expensive.
So i decided to buy bluetooth A2DP receiver with handsfree kit function (about $15), wired to AUX port and connected with empty button in dome light.This build is made in C30 but can be adapted to any car with AUX or even without AUX (if use FM transmitter).
Stage 1 was activating the dome light empty button. I will use it to pick up and end calls, play/pause music but once the button is activated you can hard wire it to garage openers, anti-hijack systems...
Stage 2 was modifying dome light indicator (fasten seatbelt & passenger airbag off) do I can see if the BT receiver is on, connected and working as it should.
Stage 3 was modifying the BT receiver to work as planned: extending the microphone and status LED, hard wiring button to the dome light, then connecting with AUX and power from 12V outlet.