The facts: 2 day old '06 package 8 - at night, with lights on, the nav screen stays bright. Dealer ordered a nav manual for me, so nothing to reference yet. The car OWNERS manual states, on page 238, that the day mode button is available in the "display" screen. It's missing in mine. Finally, it states that "if the km/h or MPH button is on, mode switching cannot be done. It remains in day mode. In this case, "DAY MODE" is not shown on the screen" Well, that sounds exactly like my problem, but I don't know what "on" would be for the km/h MPH button, as all it does is switch how the speedometer displays the speed... I must be missing something simple, but I'm a PC engineer, and pretty bright.... help?
Check your dashboard dimmer dial... if the dash lights are turned up all the way bright, the MFD will be in "day" mode. - Bob R.
I had this same problem. It was completely annoying, especially for a person who usually figures these things out very quickly. With your headlights 'ON', select the 'DISPLAY' button on the top left. There is a 'DAY MODE' button on the screen and it doesn't say off or give a clear indication of what the options are, but if you press that it will solve your problem. Of course you would also need to turn down the dimmer to get night mode to go into effect. When I got this answer off the yahoo prius chat site, I really didn't believe this was the fix. I felt pretty stupid for not getting it in the first place. I still have no idea how I could have accidently hit this button.
OK, here's the review. Car is in night mode if tail lights are on. Dash is in night mode when tail lights are on (car is in night mode) and the dimmer is not in detent forcing dash day mode. Display will follow dash day/night mode unless the Day Mode button has been pressed in the display settings. Button will only show up if dash is in night mode. Note though, the display backlight intensity follows the dash day/night mode regardless of the display day mode override. However, it will still follow the day/night LCD contrast/brightness settings.
Although Dan implied it, I'm going to out right say it... You can also set the contrast and brightness separately for "day" and "night" mode. I have my night mode brightness and contrast set for very dim and my day mode brightness and contrast set for a very bright.