On the chance you do have an early car, turn it upside down and check for 6 white tabs/nubs protruding through the slots in the floor. If these are present, you must take one more step and remove those nubs with a hobby knife, as they are actually part of the roof casting. This “feature” was discontinued quickly when it was determined the window nubs/tabs alone were enough(or a service station went to change a bulb on the first one to blow out!).