While surfing the many Mustang forums I visit on a daily basis yesterday I came across a thread over at themustangsource.com
where forum member 'mrkabc' replaced the stock Shaker 1000 head unit out of his 2005 Mustang convertible with a brand new factory Navigation unit. Up until this year navigation units were not available to order on your Mustang and are now available on most models including the GT500.
According to the thread installation can be handled with some basic hand tools and can be completed in about 1.5 hours. The tools needed are a 7mm nutdriver, phillips screwdriver and a panel puller. The installation is very well documented with high-res photos at each step. Check out the installation here