'04 cts nav question
 

I recently bought a 2004 cts with the nav system in it. and of course, the old lady who had it before me never loaded up the nav dvd, and of course, again, the dvd is nowhere to be found.

i know the dvd's are available on ebay, etc., but my question is this: i see the original dvd's, and then i see that an update was put out a few years later. if i want to have the most current maps, do i have to purchase the original equipment dvd, and then the upgrade, or does the upgrade work if i was to install it on a 2004 nav system that is basically blank?

thanks.