What is this button for the AC system-1985 -911
 

ALL,
I have a 1985-911, built date Dec 1984---I am attaching a photo of another 1985-911 NOT MINE and I am trying to find out what this button circled in yellow does.

My 85-911 came with option AC, however I have the far left and right knobs, but the middle section of my control has a rubber cover in place of the yellow circle and I have not been able to find what is specifically does for the AC system. Thanks

Those labels look like speakers. I'm going to guess it's a front/rear fader for the radio.

Yes, fader control.

Thanks! Have a great weekend!