I've only ever seen this when the mk4 has a memory fault mate (you may find that it wont save the brightness setting either).

The way that the TV, mk4 and screen interact is a bit of a mystery, AFAIK it's,

The Mk4 is used as the primary display driver device and the screen uses the defaults of that unit (or custom setting of that unit (see the nav service menu)).

The TV is the secondary display device, and will only take over if the nav drive goes off line, once off line the TV acts as if it's primary, not sure if the TV has any user setting TBH.

The screen is just a dumb terminal, and has no user settings, so is controlled by the nav unit or TV module, there are some pots on the LCD controller panel (behind the LCD), that you can use to adjust things like contrast, brightness, Green, blue and red adjust as well as scroll settings, but this requires a LCD strip down (I've done it to repair a scrolling screen), its a bit tricky as you need to fiddle with each pot till you find the right one (while its working).

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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.