In final form, a Bluetooth module and Arduino Pro Micro are stuffed inside the stereo along with a DPDT relay to switch the I2C bus.

SIM900-Arduino-Relay-Control This project uses GSM Module (SIM900) with Arduino to switch Relay state by SMS OR CALL.

you can download from Arduino Library Manager or directlly download from this repository. Connect the 8-channel Solid state relay v1.0 to Arduino board's I2C port, compile and upload ...