Extra Cooling Fan and Switch?

[N7GTX]

While fiddling about under the dash near to the boot release button, I found another switch. It is a rotary type similar to the one for the interior blower. When it is switched on there is additional cooling air on the driver's side vents only. Is this an aftermarket modification or is it something other cars have? Despite the very detailed history, I cannot find any reference to it.
Just askin'

[N7GTX]

Thanks for that.