You could use the Sonoff SV, but removing the 2 zero ohm resistors as explained in the instructions in order to isolate the relay. You would need to see how to power the Sonoff card, but it the light is 12v you would be able to pull 12V out from somewhere there. Then connect the relay in paralel with the on/off switch.
https://ewelink.coolkit.cc/?p=151
Oliver P