I know a few people have had problems removing the original Fiat tweeters from the dashboard. There's nothing like having seen the bits to know what you need to do when you're up to your armpits in dashboard!
Here are the three pieces that makeup the tweeter / grill assembly.
On the left is the grill plastic with mesh. This clips into the dashboard by lugs at either end. On the vent end the lug is rectangular and has to be located before pressing into position. The tweeter end lug is triangular and can be released by pressing it towards the tweeter. Removing the whole tweeter assembly first is probably the easiest way to remove the OEM tweeter.
Both the tweeter (right) and the spacer (middle) just twist off
I've now installed my Infinity tweeters a bit better than I originally did - I'd taped them to the top of the tweeter grills! Here they are as installed now.
The ability to aim the tweeters has given a nice improvement to the sound that I'm getting.
Have fun!