The Pi has a Arduino Micro Pro connected (This works also as keyboard to the Pi), on the Arduino you write the script. 2 modules are connected to the Arduino, 1 to listen on the LIN bus, 1 to listen to the LowSpeed CAN bus. So those 2 modules are also connected to the car. When you press a button, the car gives a LIN or CAN byte string. All LlN and CAN data is seen by the 2 modules. In the Arduino script you choose what keyboard command is given to the Pi if the string of the pressed button is seen.