Radio Harness Standard???

The matter of radio harnesses is a bit of a mystery to me.
I thought there was supposed to be a standard so you could simply swap
head units but, the last time I swapped one (for the same make) I had to buy another cable (I was swapping a Sony for a Sony in a Mazda).
I now want to replace a JVC in a Fiat (Ducato) with a Sony. I can't find details of a suitable cable. I'm sure I could adapt what I have- I have the Sony cable and the exist Fiat to JVC one and making the connections in no big deal, but I thought the idea behind the ISO standard was to allow a straight swap - or have I missed something?
Or, does the standard just specify a basic connector to the vehicle which you then need an adaptor cable for? If so, are cables with plugs and just wires available (ie no radio specific ends on them)?
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