MY IS300 has no/intermittent left channel when playing any CD, but plays through both left and right speakers no problem using either the radio (inc ipod connection through aerial jack) or the tape deck.
I asked about it on here but no solution to date. suggests a failure between the CD and the rest of the stereo but there is nothing external I can find.
So I'd check all input your sources first before you accuse the speakers or speaker connectors, just in case.