what rear speakers?
I do have rear surround speakers but they only play when I have a dolby 5+ channel feed through a totally different signal path.
So I'm guessing you have your AV reciever set to something like "all channel stereo" where it sends a signal to all the speakers. I think I've seen that on my Denon but never used it. It would be up to the OEM how they managed that so get your user manual out.
Jerry