speakers not working

the guy i bought my stang from removed the aftermarket stereo and put the factory stuff back in. the problem im having is everything is connected properly from what i can tell, but when i hooked up the door speakers the rear speakers stopped working. is this a wiring problem or a polarity issue? can anyone help me with this issue?

do the door speakers work though?

yes, the door speakers seem to work fine.

When diagnosing a problem, get a piece of paper and build a check list.
(1) Test each speaker individually using a home stereo.
(2) Test the front/rear fader on the head unit.
(3) Test the left/right balance on the head unit.
(4) ...