This may be a lot more complicated than you think.
I had a Punto Mk2, with a sub fitted from factory in one side of the boot. Seems like a good idea, until you load the shopping, and find the sub vibrates the carrier bags, so no radio on the trip home.
It didn't take long for me to want to disconnect it. That was easy, with a simple multiplug in the boot. But that highlighted another issue.
With a standard sub, the door speakers didn't handle the lowest sounds, so the sound quality was now horrid. To leave the sub disconnected, I'd need to replace all four door speakers. So I lived with no radio when bringing shopping home. Only had the car 9 months, otherwise would proabably have changed the door speakers.
The reverse of this means that to get the best from the sub, you'd need to stop it fighting with the door speakers. So those would need to be changed for special ones.
The sub was powered from a relay, I think, which was switched from the 'power aerial' output from the head unit. It is doubtful Fiat would have included the wiring if not specified from factory, so there will be a bit of wiring to be done to make it work as an add-on.
Might be good enough for you if you just replaced the standard speakers with the best you can afford, but you'll probably need adaptors to fit them - see Connects2 website.
If you play any music too loud, it can reduce your ability to hear sounds outside, which can increase your risk. Adding the sub would probably need advising to your insurance company as a modification. Repalcing the standard speakers with very good ones is not a mod, just routine replacements of 'broken' parts.