Just to add,not every vehicle came with a radio automatically back then.
Beginning in 1970,vehicles equipped with factory radios had their antennas embedded into the windshield.
I found this TSB pertaining to windshields/antennas in a old salesman's binder dated 1972.
Starting in 1970:
When no radio was ordered,nor tinted glass either,the vehicle will have a clear windshield and will not have an internal antenna lead attached to it.
When no radio was ordered,but tinted glass was,the tinted windshield will have an internal antenna lead attached to it.
When a radio was ordered,but tinted glass was not,the clear windshield will have the internal antenna attached to it.
When a radio was ordered and tinted glass was too,the tinted windshield will have the internal antenna attached to it.