Can someone tell me what the different components are for the Navigation System for a 2003 X5 4.4i?

I took the vehicle to the dealer because when i select GPS Navigaion I just get a screen that says please wait.

The dealer says I need a "control module" but from what I can tell there are 2 basic components - the computer module and the video module.

The video modulse seems fine...I am assuming there is something wromg with the computer module that holds the CD in the rear.

Am I missing something?

Also - it looks like the computer modules are available online...how do I make sure to purchase one that is compatible?