Now Ive sorted the require codes I can actually move onto making the project a reality....

Bluetooth will be handled by the Microchip RN52 module which as far as I can tell allows for both microphones already installed in the car to be used and should have sufficient output drive to drive the input to the headunit without any further analog circuitry. This just simplifies and reduces the component count.

The micro to drive it could pretty well be anything I could go down the Arduino route but I dont care much for adding to many "modules" and woul dprefer a more integrated solution, yes I could get a chip and just use it or I could go back to my roots and use a PIC....

I just need something that can send a few commands over SPI to the Can driver which will be doing all the CAN bus heavy lifting, there is no signifcant timing constraints so pretty well anything that can handle SPI would be suitable.