I know it's of no importance and you probably know already but for me it was new...

I've had some electrical issues so I'm very suspisious about everything electrical doing something strange. It cought my attention yesterday the digital readings in my 1990 S4 fluctuated a lot (bright/dim) while driving with my headlights ON. It was a bright sunny day and after a close monitoring of the pod I noticed the display became dim as I moved through a shaded area and it got brighter when driving in sunny area. The rate in which it adapted to the conditions was impressingly fast.

Now I know there has to be a light sensor somewhere in the car, but you probably knew already.

Thanks,
Paul

 

I didn't know this. Where is the sensor, anyone, bueller, bueller.

 

Yep. I've never noticed it working, but it's shown in the owner's manaul on the instrument panel page starting with the digital dash in 89 as "phototransistor for brightness control". Number 7 in this image.