It didn't work for us in the "magic" black area, but holding it out the pano roof worked okay. But that's a hassle, especially in the rain.... ;-) We haven't taken the car on a long trip since then, so I haven't tried just holding it behind the mirror stalk itself.
The problem with "trying" odd places to find something that works is that I don't want toll violations if it fails. In my experience (when we tried the area to the right of the stalk, which supposedly was the place) is that we have to practically be passing through for the thing to register normally, so by the time we know if it works or not, it's tough to do anything about it. Once I had to back up a bit to pay and an idiot was following too closely and I could barely reach to pass money to the toll booth operator. After that, we started using the pano roof and (in rain) side window.
For me, this is the only real problem with my Model S for long trips. Charging? No problem. Toll transponders? Bzzt! ;-(
On the plus side, there are no tolls between DC and Raleigh, so once a supercharger is up and running.... :-D