Dim display backlight on IQ3 display dash?

[Randy E]

I have a IQ3 display dash with a 8-pin plug on the back. With logging dash there's a wire lead for headlight signal to dim display. My dash apparently does not have way to dim. What am I missing here? In other words, how do you get dimming to work by signal from headlights? This is a race only vehicle and at night the dash is too bright, but just right during the day.

[nigel b]

I used a USM for headlight input.