For assistance with setting the screen brightness, you can find a section on that in the Tesla model Y manual:
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(relevant section to the OPs question)
Customizing Display and Sound Settings
Touch Controls > Display to adjust display settings to suit your preferences:
- Appearance: Customize the display to be Dark or Light. When set to Auto, the brightness changes automatically based on ambient lighting conditions.
- Brightness: Drag the slider to manually control the brightness level. If Display Mode is set to Auto, the touchscreen further adjusts based on both the ambient lighting conditions and your brightness preference. Model Y remembers your chosen brightness preference and adjusts the touchscreen accordingly.
- Screen Clean Mode: When enabled, your touchscreen darkens and temporarily disables to facilitate cleaning. Follow the onscreen instructions to exit Screen Clean Mode.
- Touchscreen Language: Select the language that the touchscreen displays.
Have you tried turning OFF the "Auto" setting, since it sounds like you dont want that?