Fiat Marea 105 jtd weekend sound system

I have just had a Sony CDX-T69 6 CD changer fitted to my Marea with an AD182H2 head unit. It worked for a day, then the word AUX appeared on the screen instead
of CD and none of the controls worked the CD changer. The radio still works fine, but when I switch to CD the AUX message comes back up and nothing happens. It will be a long while before I can get back to the garage who fitted it, so does anyone have any ideas as to what the problem is, or how to solve it. Thanks.

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