So it appears that it is not possible to update Navigator III maps, although I swear I have heard from some who have been able. I recently purchased an update from Garmin who just sent me this reply:

Thank you for contacting Garmin International.

I apologize that you're having this issue. In researching the device you have, this was a special order device and does not have an option to update maps. Because of this issue, I have started the process to refund the cost of the Lifetime maps that you purchased.
With Best Regards,

Software Support Specialist
Software Team
Garmin International
913-440-8280 (fax) Att: John R, Associate #5884
Additional solutions may be found at
So it looks as though I am trapped in 2006!