FM stereo speaker select switch????

[wb7sdxgary]

I have the 2006 Keystone Cougar 304bhswe travel trailer. My question is about the fm stereo which is right by the door on the left cabinet as you enter the RV. It has 3 speaker selector buttons on the stereo. I know what two of them do as they just split the music to different speakers in the RV, but the 3rd button doesn't do anything. Could it for wiring speakers for the outside??? Any help would be appreciated. I'll look at the stereo and get the brand and model number of it so I can do some better research on it at a later date. Thanks, Gary Vancouver, Wa.

[Bugle Boy]

We have three switches too. A, B and C. A is living area, B is bedroom and C is outside. Love my oldies with a nice campfire.

[BlindGuyNAR]

If you have outside speakers it is for those. If you don't have them then it's unused.