do you know some good garmin gps glare shield to reduce sun glare visor is not the option I like. Garmin StreetPilot C310, C320, C330 GPS all have this problem that can not see well under bright sunlight.
I would get a tented strip put on the top of your windshield, I have one so I can see my indash screen better. Go to your local window tint place and do it for you. Its a normal thing to have done.