My car (2009 D40) only got one when the head unit went in (the factory unit was a radio and CD player). They just put the antenna on the top of the dash with the wire running across the bottom of the windscreen and into the dash area.
My car originally had a radio/CD player, but I had them install a 7" entertainment system with satellite navigation. The GPS antenna is what I was referring to - it should have a good view of the sky, to provide faster more accurate results, so they placed mine on top of the dash.