Just pop the grille off with a screwdriver, If I remember correctly. You might have to modify the housing slightly, depending on the speakers you are using.
as long as the new tweeters have cross-overs on the you will be fine, just take of the OE fiat connector and wire to your new tweets. I have infinity reference tweets in my dash.
You will however need to make a bracket for the tweeters to sit on. easy enough to work out sizes etc once you have the speaker grille off.
the speaker grille is pressure fitted and u will need to slid something down the edges to pop it out. use a bit of rag between the screwdriver and the dah so that you dont scratch it.