Sometimes my stock radio does this, sound only from one side. It will happen when it's on aux but persists when I switch to another source (radio or CD). It usually only happens when I start the car up after its been sitting a while. The only thing that fixes it is adjusting the balance, like the balance will say 0 but sound only comes from the passenger side, but when I balance the sound toward the pside it will start coming out of all the speakers again. Just a weird glitch once in a while.