I believe the sensor for the lights is where the rain sensor is (i.e. attached to the rear view mirror). I saw the dealer cover up that part of the windshield to get the xenons to turn on in day light (I had a problem with the igniters). The point is that maybe a dirty windshield lets less light through so the sensor thinks it's kinda dark although to your eyes it's bright out....?

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2006 BMW X5 4.8is (current)
2006 BMW Z4 3.0si (current)
1992 BMW E30 325i cabrio M-tech (RIP)