The only real way is to adjust the pot on the back of the LCD panel, but this involves stripping the HU right down, I have done this and it does work, but it's a hell of a job.

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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.