I have the 430, so my advice might not help, but it's worth a shot.
play through the menu buttons, there's a button for Day/Night setting. It's the contrast/brightness control. You can have it on 'Day' setting which is always bright, or 'Night' setting which is always dark. Or the Auto which dims down when your headlights come on.
The other thing I noticed is that when I turn my gauge lights on all the way (the dial under the headlights knob). If i turn the gauge lights to the first click, which controls the gauge lights, the second click controls the dome light, but if I turn it to that first click the stereo screen is always on it's brightest.
Like I said, I have the 430, so my advice might not work for yours, but it's a start. Good luck.
I agree with Hex, rotate your your dimmer switch. It is linked in with your screen brightness. If you rotate to the right and into the first click (just before interior lights come on) your MYGIG will go into "party mode" where it is set at maximum brightness and automatically stays in daytime mode.