Hey Waylon,
I have a 2006 STR and have the identical problem, also living in S FL (WPB). It started shortly after I was rear ended, passenger side. When the car was in the shop for repairs I changed the battery. When I picked up the car I first started noticing the nav blank screen issue. Mine also seems to work fine when the car is exposed to direct sun.
I'm thinking it may either be a loose connection near the NAV CD or an inferior battery (or both).