Occupied CD changer port behind stock radio? What is it for?

For my 2005 350z, the CD changer port behind the stock bose radio is occupied. I thought that port is for Navigation and external cd changer. But I have neither one of them. Then why is it taken?

 

All 4 of these ports are occupied?

 

Only 3 occupied.


This is from wiki.

http://www.*********.com/wiki/images/e/e0/Radioback.gif

1 is for radio power
2 is for radio speakers
3 is for navigation
4 is for external cd changer

For mine, all ports are occupied except for navigation.

It puzzles me why the cd changer port is occupied.

 

basically im trying to install this Yatour digital changer. It will give me a AUX in.

http://www.yatour.net/shop/?type=detail&id=46

http://i698.photobucket.com/albums/v...MP05-MODEL.jpg