As far as the iPhone goes, I don't think there is a way. As far as a DVD player...or preferably a small multi media player with a remote, ssd card and rca out like this
https://www.amazon.com/dp/B009B5FLKY/re ... N31...that will be very easy to install. You dont have to replace anything, and you can do it yourself if you have any experience with splicing or soldering. Off the top of my head, all you have to do is tie the rca out of the device directly to the aux in. You can either read the fsm and find where the aux runs into the nav unit, or run the wires to the aux input so it's easily found, and splice/solder there.
Or look at my carputer build for how to tie directly to the monitor and leave your aux for something else.
You'll also need a dcdc converter to adjust the voltage to the device you plan on hooking up if it's not a 12v device.
Whatever you get be sure it accepts DC power. The easiest way to know is if the power cord to the device has a wall wart..some type of small box within the power cord, and that will also tell you the voltage needed by the device.