If you can't find it anywhere on your car, call Ford, provide them with your VIN, and as long as it's never been changed they should be able to tell you
+1, each car comes with a special code, a master code from the factory. You can program additional codes to it, but the master can't be erased to my knowledge. I'd call the dealer unless you you have another reason to remove the door panel.
2004 Mercury Grand Marquis LS HPP, 97K miles