Just below the diversity antenna
I bought a holder from a local car wash with two suction cups and stuck mine just below the diversity antenna, directly behind the right side of the mirror. Of all the warnings I've ever heard about tint, the only rental car I've driven that blocked the EZ Pass was the Pontiac Aztec. You could actually see a metallic layer in the glass, unlike any other car I've driven.
That being said, sure I would try putting it behind the tint, but having seen what happened in the Asstec, I would not put it behind the diversity antenna on the Mo. So long as the EZ Pass is registered to your car, they'll snap a photo and bill your account based on your license plate if they can't read the tag anyway, so just put it there and try it out!
'03 Murano Bordeaux/Latte
SL Premium AWD/Nav/AV Electronic TV tuner, Generator's NoNag, VIN etched, Belkin TuneBaseFM for iPod Mini, Sirius Sportster Replay, new CVT@63k mi.