2010 XLT correct? If so when you leave the lights in the "automatic" setting on the knob, when the daylight is just at the edge of triggering the top dash sensor, the headlights turn on and off - that results in your dashboard dimmers coming on and off. Happens on mine right at dusk. I can be driving and the dash dimmers come on (lowering the brightness) and then off (full brightness). Once it is dark enough outside, the lights stay on and the dash cluster dimmer stays dimmed.