1968 Dart vert rear speaker

my 67 had a reverb unit tied into the AM radio too. Music would get a concert hall like effect. Spoken voice sounded like head in a box.
I want to say that 1 rear speaker was mounted in the seat back frame/structure but my memory fails.
I remember throwing that reverb unit away along with its switch, bezel, and wiring, but I don't remember where that was mounted either. Right of the radio I think.