Anybody know how to fit a AUX cable in a 2004 stilo?

You can't. Only way is to connect a device that the OEM stereo accepts as a CD changer.

Or you get creative and find the pins that take the audio signal from the media (cassette/CD) and just solder on the AUX cable. As my friend did and it works beautifully. All you need is to tell the radio that the media is inserted (bending something for the cassette) and solder on the connections. Very cheap and cheerful.

Just in case it helps someone here, I found that shorting pins 13 and 14 was the trick to get my Fiat Panda 2012 62 plate with bog standard radio to go into aux mode when pressing the media button. Before doing this, the media button just kept resulting in 'no source available'.

Once you short pins 13 and 14 (CD changer data+ and CD changer data- on the pinout above), pins 18,19,20 are then active as aux in.

Interestingly, on the pinout diagram on top of the stereo, pins 13 and 14 are not even labeled.

Have any of you tried a bluethooth cd changer adapter?
ebay.co.uk/itm/Bluetooth-MP3-CD-Changer-Adapter-USB-Extension-for-Alfa-Romeo-147-156-/121722510300?hash=item1c573a1bdc:g:tJkAAOSwQTVWAR-A