With a 7" screen in front of you , I hope you`re not on the same stretch of road as I am . Why would you want to turn your sat/nav into a TV ??. On my dash I have a compartment to keep the user pack in , I find a A5 plastic clipboard in there is ideal for sticking the sat/nav on .
Unless there are some AV connections on the screen of the sat nav then I would doubt it.
If you did want tv in this area, perhaps have think about changing your radio.
You can buy Double din systems that have full screen units that can play dvd's and, with a tuner box, freeview as well.
Go on ebay and search double din to see what I mean.