Thunderchickens & Lincolns had a option for rear speakers, at very least a dealer add option that used a remote stereo fader. I have seen a fader that also slipped onto the tuner shaft side before too, but right now I cannot recall what carline it was on. It's not impossible for someone to have installed the seperate speaker harness into a Cougar or other 68-up Ford product that had provision in the rear package tray for the rear speakers. This method of an installation would look faily good as a "factory installed option", though more than likely dealer installed or P.O. installed.