Rep Power: 287

I'm trying to install the Pioneer deh-p4500mp headunit into my brother's s40sx. I get everything connected and everything, and when I turn it on, the headunit works fine, the amp and subs work, but i get no sound in the 4 speakers in his car. I tried everything i can to find out the problem, but i cant figure it out. I called pioneer and they said something about the negative wires to the speakers butit makes no sense. it bothering the jesus out of me! can sumone help me figure this out??