ok. back in town for 3 days before my next flight.
as promised, heres some pix. sorry cant be bothered taking the dash apart to show you the cabling.
DVD changer mounted under passenger seat with wiring coming up from beside the gear shift, connecting to a normal 2-way relay, and up to the screen.
Navigation GVN53 mounted just behind the 4x4 switch (you can fit a horse in there) with wiring to the same relay as DVD.
just beside the 4x4, you can see a rocker switch. the switch idea is very simple.
the relay basically has 1 control input, and 2 outputs, first output displays the nav screen always until you power up the DVD. the dvd power-on trips the relay, and switches the video to the DVD output instead of NAV.
control for the relay is tapped into the power for the DVD.
the screen remains default nissan screen until u flip the rocker switch.
DVD off gives you the Nav screen.
DVD on gives you Nirvana
ill try to do a more decent writeup next time i take the dash apart.