I had the same symptoms as those in the original post... no sound from rear speakers, but rear seat audio was not switched on and the fader was set in the middle.
You're going to have to trust me here, but there is a little music symbol on the top right of the head unit (the one that adjusts fader/bass etc). If you hold that down for a few seconds, it will reset each of those fader-style controls to the middle. I did that and suddenly I had rear seat audio. And yes, the fader *was* set in the middle before I did that.
I realise this is far too late for the original poster (and I don't think would help anyway given their update), but I thought I'd add this here because this is the #1 Google hit for "saturn outlook rear speakers stopped working".