I see from the pic that the "AUX" input is on the head, so that rules out a wiring issue on your part for the AUX section.

With your fader (and a good source like CD or tuner), set your fader to front and balance to left, then do the same type of thing systematically to ensure that you can isolate the audio to each of the four speakers.

Once you've determined that you have that working properly, repeat the process with the iPod connected to the AUX. It you had a mono jack or a bad cable, you should only get audio through either the left or right channels, but on both front and rear speakers. Getting audio though a front right speaker and (somewhat) through the rear doesn't sound like a bad cable. Either you've reversed one of the speaker leads somewhere, or the AUX section of the head unit is defective.

If you've crossed one or more speaker leads, it manifests in the audio as poor bass response...

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