If it's a smooth rotating dial, it's is a dimmer, but I'm guessing that's not it.
If its a speaker select, as it sounds, it is A-B-All.
A is dash speakers and two ceiling speakers.
B turns off the dash speakers and uses the four ceiling speakers
All is self explanatory
We have since deleted this switch. In such a small space it proved to be of little use and just added complication where none was needed.
PLEASE do not send questions via Private Message, my inbox fills up too quickly. Post questions in the forum as a new thread so that other users can benefit from the answers. Use Private Messages for sensitive information only (VIN, Contact info or things you do not want the public to see).