I have an early version of the 2004 A8L. The light sensor is different from the later 2004s. The side view mirrors get dark as soon as I turn on the car (day or night) and the rear view mirror never darkens. I replaced the light recognition sensor ($150) but that didn't do it. Any suggestions?
You have to re-boot the light sensor system. I did this by removing the fuses for 10 seconds and then reinstalling. (I did ALL of the fuses, one at a time.) My rear view mirror didn't work, replaced the sensor.... still didn't work, the a mechanic told me to re-boot) Since re-booting, mirror has worked perfectly for over a year!