95 factory stereo - no sound from the rear speaker outputs

Ive decided to put some speaker boxes behind the seat, not subs just some extra 6 1/4's i had layin around. I bought a factory wiring harness off ebay because this stereo has been boxed in my garage for a couple years and couldnt find the harness if there even was one. Problem ... when i attach the wires to the rear speakers i get no sound @ all, not even a click, pop, hiss... nothin, and the front speakers are working fine and ive checked the speakers that im putting in and they work fine on my other system. Ive never had rear speakers hooked up to this stereo either.
Does this model actually support rear speakers or are there rear hookups on 'all' these stereos regardless of the models capabilities? Model-76400