If you want to fit a standard 1 DIN size headunit, you will need an adaptor that you can get from either www.autoaudioaccesories.co.uk or Halfords (not recommended by many) and of course the genuine FIAT part which is no more expensive than any other.
Check out the Bravo Owners Organisation home page (BOO) whose link is at the top of the forum homepage. Click on the audio link and there are instructions there.