Check whether there is any voltage at the bulb holder.
If there is it is prtobably a grounding problem.
If there isn't then you have to get at the ignition switch,
then follow through to the associated lighting switch,
checking as you go that turning the ign switch produces
the expected voltages at the output of the ign switch
contact block. I've known the molded contact blocks
to cook and fail.
From the contact block there is a feed to the headlights
and other switched services (including the heater).
A basic failing of Fiat electrics is that the headlight
load, in most cases, goes through the ign. switch,
which can cause problems.
The switched output being headlights, heater fan, wipers,
radio, and a few other items, which is a fair sized load.
You check at the headlights first because it is an easy test,
getting at the ignotion switch a bit more difficult, if you
need to replace the ignition switch the job is a pain in
the butt.
Sometimes you can just replace the contact block.
If you replace the switch then think about ordinary hex head
bolts rather than the snap head security bolts that come with
the new switch. Car theives don't usually bother with
dismantling, so break head bolts are a bit of overkill.
Changing the switch can lead to one key for the ign, one
for the doors, and a third for the tank.

!! On my back in -5C with a mini maglight in my mouth, and my cheap-o
voltmeter that had the probes fall off so I just have two frayed ends
to push against, I can see the value of using a bulb and twin flex!

I actually just figured it out while doing the above contortionist
exercise. See my other response if you are interested.

Joseph