essentially the screen in your car is a normal TV, so anything connected it will show up. you can add a DVD, NAV, xbox, reverse camera..... whatever you want.
the NAV i've installed in my car is a Garmin GVN53. its a small black box that i've fitted under the dash and connected to the screen.
the rocker switch will basically switch video inputs from:
1 - your Nissan display
2 - your installed Garmin.
you could also get a 3-way switch and add a DVD player.
bluetooth is very simple, as you can buy a ready-kit and mount it in your car. however if you want to integrate the bluetooth into the built-in screen... im not brave enough to try that.