Ok, well, most people have very limited success in that one little spot (I had zero reception using it). And I have to strongly disagree with you -- the "general consensus" is that dash mounted radar detectors do NOT work well in the Model S, which is why so many people (myself included) have gone the hardwired route. I don't know about the MX, but I can't imagine it being any better.
I used to be (and still am for my ICE) a huge Valentine 1 loyalist. NOTHING I could do would get it to work inside the Model S, even when using the "uncoated area". So then I mounted it in the nosecone and installed the concealed display and volume remote modules inside the car. Still worked like crap and had zero rear radar or laser detection If there was *any way* for me to get my beloved V1 to work in the car, I tried it. And it just did not work. And I've been installing and using radar and lidar detectors for at least 20 years now, so I know what I'm doing.
There have been some people who have reported success using dash mounted detectors, but I'm sure their "success" is more "confirmation bias" as the MS windshield is coated and blocks a high percentage of radio signals.
But if you want to try it, by all means, do.