Hi,
i have also had a similar problem, mine is a 2002 (52) hgt.
Unfortunately it didn't come with the sat nav, i eventually managed to buy the unit off of ebay and fitted it. Slight snag is that i couldn't find the garla lead (speed pulse lead) spoke to a helpful chap at local fiat who told me that you had to specify sat nav at the time of ordering the car, otherwise the car will have the incorrect wiring harness in it. I then enquired about retro fitting a speed pulse wire. This was met with some caution, apparently fiats have a CAN BUS (I am sure somebody on here will explain what that means better than i) which means effectively that all electrical signals pass through just 2 wires so you need the correct wiring harness to get the speed signal to go to the stereo. He wasn't much help after this, so i spoke to a local stereo dealership who said that yes you could fit a device capable of generating its own pulse, however these are about +£80 and rely on magnetising the steel bands in your tyre.
Sorry i can't be of much help but i thought i would share the infomation i have found out. I am aware of seeing a few 1.2 puntos with sat nav and the both companies i have spoken to agree that they most likely have put in a new wiring harness.
Or has somebody else found another way?