This 16AWG blue wire runs from the radio to the rear seat 6×9 speakers.
Wire length is 16 feet. Once installed, coil and tape any extra wire to the floor under the back seat as explained in the GM assembly manual.
Factory correct wire for these GM vehicles listed below:
Buick 1964-77
Cadillac 1964-77
Chevrolet 1964-77
Oldsmobile 1964-77
Pontiac 1964-77