You could always fit a 12v powered relay/coil, when the xbox powers on it activates the relay which allows power to the monitor.
What the heck is that, can you give me a link?
A
relay works like a light switch, but instead of you moving the switch lever, a magnetic field moves the lever.
He's talking about using the Double Pole Single Throw (DPST) relay shown here.
The top of the coil (spring-looking part on left) is connected to the 12v wire from the X-Box and the bottom is connected to ground.
Connect 5v to A1 and 110v hot (black wire) to A2. (Or whatever voltages you want switched.)
Connect B1 to the circuit you want to power with 5v.
Connect B2 to the circuit you want to power with 110v.
When the X-Box powers up, the 12v passing through the coil generates a magnetic field that closes the switches. (Picture a magnet pulling on the dotted line.) A1 connects to B1, powering the desired 5v circuits. A2 connects to B2, powering the desired 110v circuits.
As long as the X-Box provides 12v, the relay keeps the switches on.
When you shut off the X-Box, 12v isn't produced any more, and the switches return to the open position, shutting off the 5v and 110v circuits.
Scott