Howdoo. Been a tad busy over the past couple of days...
Your upside down sunset button is generally for the inside lights. Sometimes wired to put ALL internals on - both of the rears, the front, and sometimes an additional one is fitted in the centre of the roof.
Can sometimes also be wired to work as an opposite, i.e. to stop the internal lights coming on if you open a door. Normally only on 'specialist' unmarked cars and when used on an 'obs' job and someone perhaps has to get out on a foot follow and the light coming on would show out.
Also heard of it being for hidden blues (such as Marks) but normally now, given health and safety requirements, a set of fully marked, independant switches is required to operate the array of christmas tree lights thats available.
I would lay odds on switching on the internal lights.
HtH
Broocie