After adding the new lines to sys.txt (and rebooting radio/restarting car), that screen should no longer show up when you reach your destination. But even if you leave that screen enabled, just don't press the "Turn off" button anymore. :-) Press "back" to get back to back to the GPS software or press the "Navi" hardware button to switch back to radio functions (Navi button should switch between Navi app and radio functions - at least that's how it works on mine). Like I said, the Navi app should always be running and you can just switch back and forth between navi and radio functions with the hardware buttons on the radio.
Unless I'm misundstanding what the issue is?
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