I still have my old Pioneer AVIC N1 nav unit that slides out. Unfortunately the screen went kaput on me and doesn't show anything anymore along w/ the fact that it's very tempermental where sometimes the screen wants to come out while at times it doesn't. I also have a dvd player installed right under the nave unit since the nav is a single din and not a double din like the stock runner radio. I have two screens behind my headrest w/ one also not working.
My suggestion to you, if you decide to go get a nav unit, get a double din instead of a single din that slides out as you'll save yourself a lot of headaches, plus you don't have to add extra parts as almost you can just slide it in to replace the stock radio. Also, if you have the funds, purchase new headrests w/ monitors installed already instead of cutting out your OEM headrests.
One bad thing that I've noticed having an aftermarket nav unit is when the screen goes out, I can't make the necessary changes to my unit as it requires the touchscreen to work it. I found this out the hard way when my battery died on me while I was injured and wasn't able to drive my car. Now I'm unable to use my iPod/iPhone wired connection to play my music on my car stereo. This is a problem since my unit was one of the first ones to come out before smart phones and mp3 players became popular and you can connect a wire straight to the unit itself.