Hello i finally sat in the back seat of my 65 malibu ss. I also found out that it doesn't have rear seat belts. Do the come with seat belts. I has the original back seat. Thanks Josh

65 back seat belts are an option. I would install them if you plan on having anyone ride in the back seat.

jk

Were seat belts front and back an option at first, and if so when did they become standard?

The Federal regulations.

(1) Seatbelts required in vehicles manufactured after 1964. (1) An automobile that was manufactured or assembled after January 1, 1965, and on or before January 1, 1968, must be equipped with safety belts installed for use in the left front and right front seats.
(2) A motor vehicle manufactured after January 1, 1968, must be equipped at each designated seating position with a safety belt system required for that seating position by the standards of the United States department of transportation at the time that the vehicle was manufactured.
(3) The safety belts required by this section must remain installed and in good working condition.