I'd be thinking the "turn Headlights on" message would be that the unit uses the light's power to power the unit and charge it's internal battery.
That makes sense as not all cars have the the aux power connected to their trailer wiring and the lights would be only continuous power available for the unit on the trailer.
And as far as "seeing" the brake lights? This would be done by sensing the voltage at the trailer socket of the brake lights and not an optical sensor that detects the light emitted by the brake lights themselves.
04 L322 Vogue V8 - Work truck
07 Freelander 2 TD4 SE - The wifes
74 Leyland P76 Targa Florio - Aspen Green
91 Kawasaki GPZ900R
Previous LRs = 78IIa series - 81, 93, 95 RRC - D2V8
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