Car handsfree

My car is older but I still don't miss anything about it. The only thing I miss is a hands-free phone.

My idea was to connect the mobile phone via 3,5mm jack to the small PCB, which split the audio signal into two. One for the microphone located in the interior light, other one for aux input on the radio. The PCB should also be equipped with electronics that will allow me to control the mobile phone.

I started with some small reverse engineering. 

I was looking for a message on the CAN bus indicating, that the buttons on the steering wheel were pressed. I found it with ID 0x5c1 and length 1 byte.

I designed PCB with jack connectors, relays simulating buttons and MCU STM32F072. 

Board is integrated into box and it is connected to radio, microphone, CAN bus and power supply. 

The board continuously monitors the can message 0x5c1. If I press top left button on steering wheel, relay on PCB is switched on. If I release button, relay is switch off. I use only function A, which allows me to control music playback in addition to receiving calls.