Do you have the owners manual for the sony
You could go online and find the manual.
I am guessing the cable is an extension because you can't see to plug into the back of the unit.
Are you sure the iPod was "outputting" when you did the test?
Can you reach in back to see if the extension is securely in place?
Was something plugged in when you tried scrolling through the input selector ?( this one was a long shot )
The extension may be after market, call Airstream or dealer
Lastly, the extension cable, plug, or unit may be faulty.
P.S. I might have said someone here will probably know, but I hate when people say that