What is this a function of? Never happened before?
 

Well, I replaced recently, an interior mirror with a B6 compass mirror and my cluster so I can finally read the center display...

I was messing around with the illumination knob on the cluster to dim down the lights, except they wouldn't dim?! WTF! BAD CLUSTER!?

Nope! I started to drive around then and when I was in the sun, the lights would brighten and when I was in the dark, the lights would dim down... It has NEVER done this before and I don't know what made it happen! Anyone else notice this?

CD sensor on the mirror maybe? There's one somewhere for that to happen.

I have never heard of this before though!? Has anyone else noticed?

IIRC, the B6 guys enjoy that feature. Not completely sure though.

Could be the sensor on top of the dashboard(in the middle of the vent).

No, that's the sunlight sensor for the HVAC.

Did other components dim when you turned the cluster dimmer? (e.g. Radio, climate control)?
 

If not then I think your dimmer control on that cluster is defective - but if so then your cluster is defective...lose lose situation it seems like. But at least it changes/dims according to ambient light.

They all change together... How would that be defective?