I fixed mine!!!

Like the others here, my screen would start dim and slowly brighten up. Also, if I set the brightness to maximum to compensate, it would "forget" the setting the next time I started the car. My battery was dying for a while, and I never made the connection to my screen.

Today, I replaced the battery, and my screen came back to life with its old brightness. What's more, it even remembered the brightness setting that I had long ago. So, if your battery is old, you might try a new car battery to fix this.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input