If they have already built it, then I can see why they wouldn't refund the deposit. But as it's still another couple of months to go before delivery, it would seem they haven't yet. In which case, it seems unreasonable that they not refund you when they've almost doubled the agreed delivery time.
The question is, do you have any evidence (perferably in writing) of them saying that it would be 10-12 weeks. If so, I would've thought they were in breach of contract.
But, if the agreement was just a vocal "it'll probably be 10-12 weeks" then you'll just have to wait.
Seems a bit odd why it's taking so long though, I thought 10-12 weeks was the standard wait, so what's the cause of the delay?
I suggest making sure you speak to the dealer's head manager/owner, to make sure it's not just a case of the salesman forgetting to actually place the order, and him placing the order now he's been chased on it.
If no luck with dealer manager, escalate it to Fiat. There's a contact us section on their website that they usually respond to within a few days.
BTW, When did you find out it wouldn't be ready until of January? Was it just before you were expecting delivery?