I am assuming that you use garmin onboard. Which version that you use? But whatever it is, first try to edit the "hosts" file under \etc, and add following lines at the very bottom:
(note: it will only works if you have your iPhone jailbroken already because you will need to edit some of the file)
[hide]127.0.0.1 [Only registered and activated users can see links. ]
Install your garmin application with above modification and directly run it without respring/reboot your iPhone (and see if it works).
There are however, several version that have a non-functional (at least not fully functional) garmin-nav file so you have to:
[hide]- Find another garmin onboard with the same version that already been "edited" and copy the garmin-nav file under "var\mobile\Applications\[APP DIR]\garmin-nav.app\"
- Copy it to your intended garmin application to use
- Change the file permission and owner as follow:
Owner: mobile; Group: mobile
User: Read, Write, Execute
Group: Read, Execute
World: Read, Execute
Just try that and see if it helps.