UZJ100

He's looking for a way to have it bright in the day and dim at night with the headlights always on and without having to manually adjust the brightness. Unless there's a photo/light sensor controlling this the easiest way is to toggle the illumination wire.

Can't seem to find anything in the manual. It would be odd if the only display option is "Preset 1/2/3" etc.

Really? The illumination wire allows you to have variable brightness? Thought the illumination wire was a binary signal to the HU, either on or off.

You can leave the illumination setting on day (vs auto), but then at night it would be pretty bright. Really not that difficult to flip a switch for what you want. If your skilled enough, and it means that much, you could prolly make a photo sensing switch.

Put a toggle on the illumination wire.