The car has a navigation system which I use all the time. When operating during daylight the system will all of a sudden dim the screen so I can barely see it. I set the light control to only daylight and it still does it. I set the screen brightness and all the lights to the brightest setting and it still dims. The service manager at the dealer took the car home to replicate my complaint and was able to do so. He searched Buick technical bulletins, etc. and could find no similar problem reported and said there was nothing he could do. I have lived with this problem since and no w have almost 30,000 miles on the car. I think it is the little dome on the dashboard near the windshield which I believe controls the lights. It may be malfunctioning. This has been very dangerous many times when driving and trying to find my way particularly when the sun is bright. You can't read the screen without leaning way forward and squinting while trying to stay on the road. I hope someone else has had this problem and has found a solution which I can use.