Other way round. Main board generates that signal to say it wants backlight on, and power supply turns on backlight.
That's what I said. We are in agreement. I think you misread or misunderstood.
OK, at this point I think I will begin by fitting a 5 volt relay to short that signal to ground. that way the activation is completely isolated. Once that is tested and working I will design a circuit that detects mouse activity and activates the relay after a certain time of no activity and keeps it activated until it detects furter activity.
It can be an extremely simple circuit except for the timer which will require some kind of counter because it needs to be anywhere between 10 to 30 minutes. I can do it with a 555 and a counter. It does not need any kind of precision.
Come to think about it, the monitor backlight should come on when the monitor is first turned on, without need for mouse activity. Or maybe not? i need to think about this.