I did a remote face in my firebird years ago. The first i had setup was useing fiberglass and blondo for a pod type that was above the rear view mirror.
The second time I flush mounted it in the visor by cutting the foam and cardboard/wood backing and making a frame that it fit in and bondo for a super tight fit and so it wasn't viewable unless the visor was pulled down. Made for awesome stealth.
But on the communication side some of the headunits can take only so long of leads. My alpines volume nobe used pulses instyead of resistance so i had to move it closer to the actualy unit in the console or i had no volume control.