IMO you do not want the one that FR would have put there at the factory anyway. Get a low profile one or it will be the highest point on the roof, looks better too. Any camper store can provide you your options, I had camping world do mine. It will need to be a unit with it's own controls as the prefab wiring is not part of the main system, only the power wiring is pre-feed to the box in the ceiling next to the vent.
2011 GMC 3500HD 4x4 Crew DRW Duramax
2012 Sierra 365SAQ
2006 Police Roadking FLHPi