On the '07 and earlier models the best unit to purchase is made by TV & Nav 2 Go. Here is the link: http://www.tvandnav2go.com/
I had this unit on my '04 QX556, it worked great. It is a little bit to install, but I guess now they have a more plug 'n' play model. Mine I had to wire in at the back of the Navigation DVD rom drive that is located under the passenger seat.
2013 Infiniti JX35 AWD (Midnight Garnet / Java) & 2013 Nissan Maxima Sport (Pearl White / Black)