The Connect Nav+ radio code is stored in the car's body computer, so if you're refitting the original one into the same car, it won't ask for the master code.
If you're fitting a different CN+, you can get the code from a Fiat dealer, providing you have the VIN and registration number of the car it was originally fitted to. You may also need poof of ownership. Fiat dealer charge between £15 and £35 for the code.
There's no limit to the number of codes you can try for the CN units, so you could step through from 0001 to 9999 until one is accepted.
Only other way to get the code is to send the CN to Magneti Marelli in Italy. Last time I enquired, the cost was €180 (EUR) plus the postage cost to Italy.
.