If you wanted a rear speaker added after delivery there was a couple of generic Ford options the dealer could slap on.
From what I recall
One was a rear speaker kit with the metal speaker grill and surround the other was the reverb kit that included the same speak, a controller that bolted on to the bottom of the dash and a small cardboard and metal box (about half the size of a tissue box) that would get screwed in the trunk somewhere.
Sure others can or possibly will post ads for one or both kits.
Of course the dealer would do just about anything to make money including selling and installing an aftermarket mono-tone radio of the period in the car.
