what is the lower stereo controller for

my brother just purchased a 2001 nissan sentra gxe touring! the question is -there is a normal (factory) single disc cd player in the upper dash location, but in the space below the a.c. controls there appears to be a multi disc changer controller???? the control buttons are illuminated when the headlights are on but the LCD screen does not come on??????it is also a factory unit. is this unit just pre installed by nissan for an in trunk CD changer (that doesn't exist), or what gives?????

I have an 02' Spec, that has the in-dash 6-disc changer.
Exactly as you desribed. It doesn't do anything unless it has a disc in it.

When the car is on, press one of the numbers.
Does the LCD come on, and say "Insert Disc"?
I think the little light above the buttons is orange when there is a disc in that slot, but not being played. When being played the light turns green.

In mine you still control the volume with the head unit, but advance tracks with the changer.

Again, this is all speculation as I am not sure if you could get the changer with the 01' model.

Hope this helps.