How? If the mirror was in the center of the windshield like other cars then all you would see is a reflection of your forehead. Unless you are really short, in which case you would see the back seat headrest. And if you folded the back seat down, you would see the inside of the rear hatch.
If you need to see behind you like that, maybe get an extra mirror installed on the starboard exterior, roughly in the location of a standard rear view mirror. But really, I think as soon as you sit in one you'll see how unnecessary it is. Below is the P3, but these angles will be true for the production car as well.
The Same Technology that's used in Rear View Mirrors inside of Motorcycle Helmets. That's how.