Yes there was a factory option to have rear speakers.
Also the blower type rear defrost uses one of the 6x9 holes to pull in fresh air from trunk to blow on the window
Ok, that makes sense. I'm assuming you could only get 1 6x9 in the rear and the other cut out would be used for the blower? Guess you would need to have a radio equipped with a fader switch...AM/FM? If anyone's got some pics of an original set up, post em!