The stereo code should be on a little paper along with your manual in
the original FIAT blue booklet. Mine was in the glovebox when I bought
the car, even if it was a used one.
Alternatively, you wll have to send the stereo to Blaupunkt (I presume
this is what you have) in order for the code to be reset. I really
don't know why you have to put the code in your stereo, since I had to
change my battery (2000 model) in my car last December but the stereo
never asked me for the code, despite the fact it is the original
in-dash Blaupunkt system.
I would try to get in touch with Blaupunkt to see if they can help
before going to FIAT.