Hi- I don't know specifically about the CD player in the Multipla (I have a 10CD autochanger fitted in mine), but my son recently got a new PuntoGrande, and I wondered the same thing.
The unit in the Punto is a single CD slot, AM/FM radio.
If you cycle through the inputs, there is: AM;FM;CD and AUX. I assume the AUX input is for a CD multichanger.
I took the radio out and there is a socket for a multiway plug from the CD changer to plug into. I phoned the local Fiat dealer and asked if there was an adaptor to enable an iPod to be connected- "No Sir, not possible, try (local audio specialist)".
I took the radio to them and asked for an adaptor- "No Sir, not possible with this sort of head unit, the CD changer sends a special signal to enable the AUX input. Try one of these FM transmitters instead". Thanks but no thanks.
So I went home, connected up the radio, switched it to AUX and prodded each of the pins on the multiway socket in turn. Sure enough it didnt take too long to find the two pins that are the left and right input.
I cobbled up a lead with a 3.5mm plug on one end, and some tiny push fit connectors for the relevant pins on the other end, hot glued them in place and that was it- instant iPod connection.
If I could be bothered I'd go back to the audio "specialist" and offer to tell him the amazingly complicated way to enable the AUX input, for a small fee....
Any way if you're at all handy- have a poke around the socket on the back of the radio, I'm sure the unit in the Multipla will be similar.
hth
Neil