I have actually done a lot of research into this, it is my job. Literally The best place to mount the microphone is to the headliner, centered above the rear view mirror. This will result in the best voice pickup with least amount of road noise. The headliner helps to dampen and isolate some road noise, and you need all the help you can get, these cars have a LOT of road noise.
I did center of headliner above the rr view. Easy since already had that apart to do compass/temp/homelink/maplight/autodim rr view install. Wiring routed in same place. Also my take was sharing the mic with passenger. Both me and the wife participate in a road call on occasion. No complaints about noise but that would depend on the mic to a large degree. I'm sure they aren't all created equal.