Hey, from my limited knowledge, the nav screen has two different versions of the dvd unit
Yours might be different to mine, so I'd suggest turning the ign on and pressing all the buttons lol. If nothing happens it's likely dead- many are disconnected when imported or have died overtime.
mine looks like this - it has an OPEN/CLOSE button but no aux input
As for what can be done with it-It should have a TV tuner behind the passenger side boot liner with AV ports
I've managed to get video playback from a camcorder through them, which can be viewed on the pop-up screen through the 'VCR' function (if it works). You could also buy a HDMI - RCA converter on ebay if you wanted to plug a HDMI input in. You just have to note the screen is in NTSC format not PAL. If you had the $ you could look at buying a jap car cd player or a HKS camp to plug into the AV ports. Those with dead screens have also gutted the unit and replaced the original screen for an aftermarket one something outside my knowledge.