The aerial on my Y-reg Multipla ELX 100 16v snapped off, leaving part of the threaded section in the base. I managed to cut a slot in the top of the remaining part and unscrew it partially before the rest of it broke off. It's still stuck in there, so I ordered a complete aerial and base unit.
Here's what I did. If you have the same model, you're wasting your time following this, but I understand that other models are different and this may help.
I removed the two caps at the front of the courtesy light cluster and removed the screws. Then I had to prise out the mirror control box and the Volumetric Alarm Ultrasonic transducer box from the front of the courtesy light cluster. The ultrasonic transducer box was really tricky and I ended up getting the Elastoplast out after a screwdriver went into my finger.
Two more screws and the courtesy light cluster comes off. You can unplug the various electrical connectors to move everything out of the way.
It find it ridiculous that it is so difficult, because you have to go through this rigmarole to replace the bulbs in the courtesy lights.
There's a black plastic tray inside the aperture. There are two more screws holding that in place, mounted almost horizontally. I removed those, but that didn't help much. The plastic tray moves only a few millimetres once undone. Sadly, it doesn't move far enough to get access to the underside of the aerial base.
End of the line for my repair attempt.
Although what I bought was the genuine Fiat part, it's not the same as the one already fitted. The new one lets the aerial pivot down. The old one has it fixed in place.
So, for my car at least, what Sicone said seems to be true; you have to remove the complete roof lining to get access to the base of the aerial.
Anyway, I'm going to see if I can either drill out the old stud, or borrow a broken stud remover tool to get it out. Then I should be able to simply screw in the new aerial.