A friend of minejust bought a base model 2008 Sonata GLS. It has only two speakers, one in each front door. The back doors have grilles for speakers but I am wondering: 1. Is there a wiring harness in the back doors just waiting to be connected to speakers. 2. Is the stock radio in this model Sonata capable of controlling the "Fade" between front and rear speakers?

I note that while the radio has a Fade control, it does not turn off or reduce the volume of the front speakers when I turn it full to the rear speaker setting. It appears to be disabled.

 

I think you may be mistaken. I'm quite sure there is no Sonata model that has only two speakers. My 2006 GL V6 has six speakers. I don't know where you are located but in Canada all models have at least six speakers.

Perhaps the fade control has been adjusted completely to the front and is no longer working?