Just thought I'd add my thanks- this technique worked for me.
I recently got a 2007 G35X without navigation, and added an aftermarket backup camera, plugged into the aux video jack.
Cutting the gray wire and grounding the blue wire back into the headunit made the camera usable, finally!
For the record, I had the battery disconnected the whole time I was working.
One small difference- the wiring harness on mine looks slightly different than the one on Mikey's in that there are fewer wires.
Wonder why. Anyone know?