The NAVI03 unit is the complete unit as shown on the picture (post by rick). This Unit has inside a DVD-rom, a computer and also a GPS hardware unit. The NAVI03 unit is connected via fiber optic to the MOST network. If this unit does not work, you may have no screen, radio failures (reset), BMW assist functions will not work, etc.
Previously I replaced the DVD-rom unit only, since I was suspecting that the laser is worn out. You can replace the DVD-rom or the laster only, search for the threads in this forum.
If this does not solve the problem, next is probably a navigation comuter failure (typically due to moisture and condensation).
That is why I ordered a second-hand complete NAVI03 unit on eBay. Thus, not just the DVD-rom, but the complete box. You can find one for 400-500 Euro. This solved the problem. It is important that you know which HW / SW you have and what type of NAVI UNIT you have (1, 2, 3, or 4). If you buy a similar version (second-hand), the software - firmware can be easily upgraded to the latest V32 (you need a special CD with the software).