"Stay Awake" is only for charging, as it says in the description, and is only designed to keep it from sleeping, not dimming. To keep the screen from dimming, you'll have to find a navigation app that prevents it. I use Waze, which prevents dimming by default. Google Navigation does prevent dimming, but Maps alone does not prevent dimming or sleeping. I've never really let the Maps thing bother me, as I usually leave Nav on until I'm close enough to my destination to not get lost.
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If it ain't broke, use a sledge hammer.