My 71 wiring diagram shows a rear speaker connector going into the instrument panel stero component wiring, although it indicates 4 wires, not two. I'm thinking that's what you're looking at.
Didn't the old radios have a separate vol. control for the rear speakers? If so, maybe yours didn't come with rear speakers or it was set up for only one rear speaker.
Here are the broadcast sheet codes I found that mention speakers:
R31 - Single F/R speaker with R11 (Musicmaster AM) Dual rear with R35 (AM/FM Multiplex)
R32 - Dual rear speakers
R34 - Premium speaker package
R44 - Premium speaker electronic