Just purchased a Fiesta Finesse (S reg, 1.3 petrol manual)

It has the basic radio/cass in it (I don't have the code!) and I'm thinking of replacing it with a radio/cd harvested from another car. The newer radio/cd is a 4 speaker system

Can anyone tell me before I have to go digging about...

• There are no rear speakers in my fiesta. Where is best to put some?
• Will the wiring loom already have connections from the radio to rear speaker positions?
• If not, will my Radio/CD get upset if rear speakers are not connected?

Regards

Andy