Ciao Davide,
the tweeters are only accessable from under the dashboard. Sorry.
But if you want to upgrade your system anyway, leave the OEM-Tweeters where they are and put new ones where you like it. (perhaps mirror triangle)
If your door already has the fixure for speakers, there will fit 6" Speakers with a mounting depth of approx. 50mm.
Check out these (similar version should be avalable in Italy):
...at least the dimensions of this set will give you an idea...
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