Have a look at the photo here.
See the connector ( unused on my car ) by the annotation number '3'. It's called in the workshop manual G313. On your car that may be in use. If I'm right and that is the case, then only the orange wire will come out the other side. The fat white/red wire won't go anywhere. It's permanent live and fused at 30 Amps ( fuse G255 ) and should be unused ( according to the wiring diagrams ). It is used for the heater fan in early phase 1s with
If you have the other half of the connector and the White-Red is unused, all you need is a suitable crimp connector like the one shown in the top three photos here :
6.3mm Female Terminals and Covers: Polevolt Ltd
which you should be able to get from a good car accessory shop/factors and crimp or solder it on to a good bit of wire. Disconnect G313, remove the plastic terminal locks off the connector, shove in your new connector and replace the locking bar. The other end then goes to the input terminal of your relay.
If you don't have the other half of the connector ( like me, it is totally unused ) and you want to do it neatly, you can get the other half from another GTV ( the same connector is used behind the radio for the rear speakers or can be found behind the dash in Fiat Cinquecentos ( probably Seicentos as well ) and Fiat Brava/Bravos ( late 90s early 2000s versions )
Or just tap into the wires with scotchlocks!
Next you need a switched supply. As it's only going to be driving a relay coil you could take that from the orange wire in G313 ( either tap into the output wire if it exists or use another one of those crimp terminals if it doesn't and you have the other half of the connector ).
Earth the other side of the coil and take the relay output to the radio.
Like you I cannot remember where other threads have suggested taking a switched supply from, but I do recall being a little unimpressed with the choice. Car radios can take a fair amount of current.
In summary :
G313 White/Red to relay input
G313 Orange to relay coil
Relay coil to ground
Relay output to radio.
I hope that's clear. If you want I can draw it out for you.