Dimmer is set via the LCM and nav screen sensors, both must detect that the ambient light level has dropped, if its not working then one of the sensors has packed up, I test mine once a month (I don't do a lot of night driving).

To test put your thumbs over the sensors on the nav display and LCM, wait 20 seconds and release both at the same time, the screen should detect the change and go bright (as it had slowly dimmed).

Don't forget that even with it on max dimm, the screen is still bright at night, have a look at nightmode, it was designed with this in mind and does reduce the amount of glare at night.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.