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When you checked the fader did you get an indication on the radio that your input was received? If it didn't show that you were changing the fade amount then the problem would have to be in the radio.
If it did show fade changing but you get no sound then the problem could be the radio, speakers or speaker wiring. There isn't an external amplifier used on this model so that simplifies things a little.
If both rear speakers went out at exactly the same time then it's more likely to be a radio issue, but I would still test the wiring for an open or short to ground and then try a known good speaker if the wiring checks out ok. If the wiring is ok and a known good speaker also doesn't work then the problem is in the radio.