On a white label which is stuck onto the radio casing. You will need to pull the radio out from the dashboard a couple of inches in order to be able to read the label. You'll need a set of radio removal tools in order to be able to do this.

Once you have the serial number then you can work out the radio code by using: www.pumapeople.com/decode.php

This is assuming the serial number begins with an "M". if it begins with a "V" then the code will only be available from a Ford Main Dealer, or there are people on Ebay who claim they can do them for a fee.