That switch is for the air purifier on the rear parcel shelf, yours may of been fitted with it, its a factory option, someone recently posted all the factory options, if the previous owner tried putting speakers in the rear they prolly ripped it out coz its fu(king annoying, as it inhibits you from removing the parcel shelf, unlike most other electric items its power supply isnt on a nice unclippable plug setup.
But yeah, thats what my switch does, fast and slow speed for big vs little fans....