Ford 2003 Mustang Workshop Manual page 46

Index Card
Place an index card or a piece of paper between the weatherstrip and the sealing surface, then close
the door. Slowly withdraw the index card or paper after the door is closed and check the amount of
pressure on the weatherstrip. There should be a medium amount of resistance as it is withdrawn.
Continue around the entire seal area. If there is little or no resistance, this indicates insufficient contact
to form a good seal. At these points, the door, the glass, or the weatherstrip is out of alignment.