If your new indash has a monitor; then;
Your new "indash" should have two or three "av" inputs.(video/audio), find the output from the factory dvd and use y adaptors to y the audio/left/right and send it to the inputs of your new head unit.
If your only talking about a new "indash" that plays dvd's and doesnt have a monitor, make sure your new unit has a aux input to which you would connect just the audio from the rear dvd unit to so that your head unit controls the volume, if you head plays dvd's it will have output just like the rear and you can send its output the the same monitor that the rear is going to. In a nutshell, unless you are splitting the vidio signal more than 3 times, you can use y adaptors, when you need to split it more, you need to pick up a amplified splitter from a audio dealer.