Honestly its a bad idea. Driving and watching telly is dangerous. I type from experience!
One day, at some lights I got so carried away I didn't realised the car in front had driven off, totally out of sight!! I was too involved watching the film. The horns of the traffic behind brought me back to my senses!
Even in normal driving, you have a tendancy to give it too much attention. Which I did! Even when it was for the benefit of the passenger I had a tendancy to glance at the screen.
I won't be doing it again (even if my non-nav car did allow me to!).