Sat Nav Advice Please

[Dakes55]

I recently bought a 2002 A8 D2 which has a Concert II radio fitted with a CD boot changer, I am a bit confused as this car has a GPS Antenna on the boot lid which makes me think that it used to have a Sat Nav Fitted, I would like to either put the car back to original specs or fit a new Sat Nav, would it be better to fit the original type of sat nav which I think should be a Navigation Plus, is there much wiring to do and are there any extra modules needed, any advice on this would be very welcome as i am not very familiar on this subject