Is it another Ford unit or an aftermarket one? If it is aftermarket you will need an ISO lead to connect it up with.

As for the speakers, really can't remember where they were in my old mk4 fiesta, they might have been under the plastic trim either side of the parcel shelf? Wiring should be there, just look under to see if you can see any mounting holes or wiring. The new head unit will work fine with only the two speakers though