The picture you show looks like a (two-part) standard ISO connection.
It looks as though your Ducato is one that came from the factory without a factory-fit radio (which would have had a Fiat-specific connector) but with radio-prep (no radio, but speakers and preparation for an aftermarket radio with ISO connectors - it's a fairly common option and the radio may have been fitted by the converter if it's, say a motorhome).
I'm interpreting that you're trying to fit a Fiat "factory" radio?