Interesting link and "how to" guide there.
Although they do recommend
"For optimal voice quality and lowest installation cost, we recommend the position at the top left corner of the windshield. If an invisible installation is preferred, the microphone can, under certain circumstances, be installed in the interior overhead light. There are additional steps needed for this option, detailed below.
When the microphone is properly fitted, a very good sound quality can be achieved. We guarantee the best sound quality but only when the microphone is fitted in a visible location. "