Navigation unit not recognized?

Just picked up my V8 Touareg this afternoon. First thing I tried was the Navigation unit. I popped in the CD for my area and the message displayed was:

Navigation system is being activated.

After a minute or two, the following message is displayed:

No navigation unit recognized.

Can't do anything with Nav at this point. Popping CD in and out just gives No nav unit message again. I know it worked when I test drove the vehicle last week! Any ideas?

Thanks