stop iphone from dimming

I'm looking for a way to stop the iPhone from dimming after a short period (specifically, when navigating via Google Maps).

I've tried turning Auto-lock to "Never" but that doesn't seem to do quite what I want, or at least not all that I want: it prevents the screen from going totally black and the iPhone from locking, but the screen still goes unusably dim after a short while.

Yes, tapping the screen brightens it back up, but the tap also stops navigation scrolling (I have to hit the "Resume" button); two taps like that are an unnecessary distraction while using it for car navigation. The only slight workaround is to touch the ETA bar, which instead causes the bar to switch between time left/time of arrival/distance remaining, but again, unneeded distraction.