It can be done...did it to mine. Here is the web site for the switcher box:
http://www.avelectronic.com/products.htm. Keep in mind, you will need to install a switch on the dash that will take the place of the "park brake switch" This will allow you to switch, on command, between the Nav system and the DVD output. Works beautifully on my car. Somebody tell me how to post pictures and I will show you what I mean.
Oh...as far as the switcher box...all you need is the cheaper version. The more expensive version has additional AV inputs that are normally on the back of the DVD head unit. I got the cheaper one and it worked wonderfully. Technicall support was great too!