Your not searching the forums, I answered this question sometime yesterday I think.
It all depends on the screen. Some screens come with a extra wire that connects to the reverse bulb. When you go into reverse and the bulb lights up, the screen automatically changes to A/V 1 or 2, then back to PC when the power to the reverse bulb is cut. This all depends on the monitor you have. If it does not have that, you have to come up with something that switches the screen for you. I tired to come up with a relay system but it never worked out.