'05 Mustangs are only made in one place.....FLAT ROCK, MI...... The month/year it was built should be on the placard inside the front driver's door on the body door sill.
I am just a little bit curious where you did look? Not trying to start anything of course, curiousity is just getting the better of me...
Window Sticker The build date on the placard is usually something like 12/04 - the lower right box of the window sticker (if you still have that) will probably show something like 12 13 04 - no dashes or slashes.