1st you will need a special removal tool that slots in the sides of the radio  to get the orignal fiat radio out 
you will then need an adaptor plate to fill the big hole in your dash board ..your new radio will fit in to the adaptor plate 
the wiring on the fiat is standard ISO most good quality aftermarket head units use ISO conectors so no problems there
                
 TONY...
                
 January 2005