There was a thread about this somewhere else where someone had used the same hole as the Speedo. But there are loads of wires that go from the engine bay into the cabin, find where that is, and you'll be able to fit your cable through with them. Haven't done it on a fiat, but on most cars there is a big rubber seal type thing where the loom goes through. Try looking in the haynes manual (if you've got one) for it. The main battery connection to the fusebox must go through somwhere.
EDIT: If all else fails... you could always drill a big hole!!!!
pull back the carpet directly under your glove box, have a torch handy to get a clearer idea of what you are looking at, there is a 4cm diameter rubber grommet with a few wires running through. poke a hole for your wire and feed it through, simple There's loads of room for extra wiring, mines got 2 bits of 8AG, my lighting kit wires and a braided hose in, with room to spare