Auxilary RCA inputs have no volume control
Your problem stems from the fact that the CDA-7892 is not equipped with an RCA aux-input jack. If you're plugged into RCA connectors at the back of the head unit, they're outputs, not inputs. The sound you hear is just feeding back through the outputs and into the head unit's amplifier.
If you want to play your iPod through this head unit, you need to purchase either the auxiliary input adapter (KCA-121B) or iPod adapter (KCA-420i).
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