The wiring ypu have highlighted is from the stereo to the amplifier. This has 4 channels. The wiring from the cd changer to the stereo is on the left and has two channels. CDs have two channels of audio, it is the stereo which is converting this to 4 channels snd sending it on to the amp and then the speakers.
I would first check balance and fader settings on the stereo, obvious I know but they could be different on different sources for the stereo. If the other three channels are working and all four work on other sources it points towards the stereo to me. You could disconnect the outputs from the cd changer and swap them over just to check what happens. I am not sure what the stereo does with the two channel audio to generate the rear channels.
MC