I replaced the head unit in my Fiesta, you can get a fitting kit to convert the Ford fitting to DIN standard including wiring harness, aerial adaptor and blanking plate off e-bay for around £20.
One thing I have noticed about the cheapo head units is they get REALLY hot and in summer I have difficulty playing CD's on mine. In the end I hooked up a 12v fan off a PC to get some airflow down there.