Hi!
I had the same problem with a customer a week ago. The radio comes from the factory with an anti-theft code. Every time you run out of power to the car, the radio will lock up and ask for that code. If the radio is original to your car, I will explain how to get that code:
1. try looking in the car's papers.
2.Go to the Ford dealership (I don't recommend, the customer asked 40$ for the unlock).
3. Apply the same method I used. You have to take the radio off, and on the radio you will find a label with a serial number. You can find a lot of websites on the net that offer radio unlocking services, I got it from here : ford radio code, or you can try ebay.
Good luck! :)