I was messing around with the radio on the way to San Diego today (long drive) and I think I just figured a way to have NAV work with Dashlogic.
As most of you know when you start the car up with Dashlogic it pops up the info on the DIC, HUD and radio. The radio info that Dashlogic shows seems to interfere with using NAV at the same time.
Try this:
With the car running and Dashlogic is up and running....Go to your main MyLink screen and select "Quick Info". Press that and then press the "OnStar Turn by Turn". Now you'll notice it will grey out after you press it. Then go back to you home screen and select "Now Playing". You notice the Dashlogic info is gone!
Now go back to the main screen in MyLink and select "Destination" and key in where you want to go. The NAV screen will pop up and even work on your DIC and HUD alongside the Dashlogic info.
This also allows your album art and radio info to use the full display again even if you don't use the NAV.
Enjoy.