Well, big thanks to
bpd138 for looking into my small problem (R cam not automatically switching when reverse in engaged). He suggested that I should look at some A/V setting before taking things apart. One of the setting was "R-Cam interruption". From the factory, "R-Cam interruption" is turned off. I simple switch it to on and voila, it works like a charm. Now, if I put the gear in reverse, the rear cam activates. I can still press and hold navi button to turn on rear cam manually.
YouTube - 370z backup camera
My installer Matt did a really good job. Everything is working 100% now!