The button that is shown on the screenshot turns the cab light on on the right side of the cab (I'm not sure why). I also don't know where the button is to turn on the main cab light.
Funnily enough I was just on ECW and I had the same issue. Appeared to be stuck on on and wouldn't turn off when I tried.
Below the desk on the left side, just about at the left armrest if you’re sitting in the drivers seat.