This is an update for anybody who has a newer second generation IS. I have a 2012 IS250 with navigation without ML. I have just successfully performed the rear camera hack with a switch, and will show you the right corresponding wires below. It is slightly different than previously described in this thread..
As mentioned before remove the head unit, and then isolate the J100(A) connector circled below. It's the big white connector on the lower right aspect of the head unit as you take it out. The one circled in Black is the correct connector to work with.
Now once you've isolated the connector, the blue wire on the farthest aspect to the right (left in the second pic) next to the red wire is pin number 16, which controls the feed from 12V going to the rear camera display.
This is the wire you must send a 12 V supply in order to display the rear camera.
I accomplished this by not cutting it but splicing into it and then placing a diode just before so the rearview camera still works well in reverse and doesn't put overload signal in the opposite direction. Find a 12 V connected to the ignition ( I tapped into the cig lighter since it's fused) and place a switch in between to control when the 12 V is sent to the blue wire. After this you are all done and it should work whenever you push the switch. There are no glitches and everything works and goes back to stock when the switch is off. I hope this helps
poorly drawn schematic I used:
And the oem-like switch I used:
Edit I got the switch from eBay.