A friend of mine has a 86 camaro that doesnt even have rear speakers.It has a piece of plastic molding that covers the place where the speakers go,it doesnt even have the speaker mounting brackets. And its all original.
as for earlier models they didnt make a 3 speaker sound system that im aware of,so if its a car with a single speaker grill in the dash,then it probably didnt have any holes cut in the back for speakers,so any holes would be custom if they are in the rear dash