I checked the manual but did not see it. It seems to be an addition made by the previous buyer.
I also asked my mechanic but he does not know what it is.
It looks a LOT like the attached photo, which is purportedly a switch which allows you to turn on a TV while driving. This would be part of an aftermarket AV system or an add-on for your current system (not quite sure).
Images and information obtained from this site.
EDIT: Here's another site which may also be of some use, though it is in Japanese. If the button is what it seems to be, this would be the site of the company selling it.