Originally Posted by 03glgold
I could be wrong, but it sounds like the amplifier in the stereo isn't putting out enough power for the other speakers.
I would have to agree with this statement. At first, I was thinking it was the speaker outs, but once you said that all speakers work individually, I ruled that out.
I am very iffy about this, but, as a temporary fix, and depending on how many ohms your speakers are, have you tried connecting two speakers, to one of the speaker outputs? It will not be very loud, but at least you will have sound coming from both left and right, rather than one side. I think I have read that your speaker load should not be under 4 ohms per channel though. So connections will be based on how many ohms your speakers are.
If still under warranty, give customer service a call. Maybe they can fix your HU, or send you a new one. This would be your best alternative.