I tried fiddling around with that, too (a little to much), and didn't get anywhere with it. However, if you have Android Auto installed, you can set the app on your phone to automatically dim the head unit. The instrument cluster won't dim, but your head unit will. If you have Developer Mode enabled, you can select Developer Settings from the app's main settings menu, then select Day/Night, then Phone-controlled as the preferred option. From then on, your phone will automatically dim your screen.