What does this button do...

Small black sprung push button ie doesn't stay on or off. I can't work out what it does. Its on the LHD rear pillar but not on the right. Wondering if its something to do with the entertainment system the last owner fitted??
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on some markets, mostly Asia the A8 has an ionizer for the rear a/c.
the switch is right in that spot but don't know the actual shape of the switch.









