Problem w/ '91 maxima's bose system

I just bought a 1991 Maxima, w/ the Bose system in it.  It's nothing special, and all i want to do is hook up my Clarian to it.  I took the CD player out of my Sentra and put it in my Maxima, (they are the same year, and so the harness should be the same, according to the local car audio shop), it works fine, the only catch is the back speakers don't work.  I checked the connections, and they're right, so i don't know what's wrong.  Has anyone else run into this problem?  How do i fix it?