2002 Focus 6 cd/radio in dash

I have been trying to help a friend trouble shoot why his stock radio will not work. CD's Play fine, AM comes in somewhat but no FM. The radio will seek and find stations but no sound comes out of the speakers not even static. Any ideas? I am also curious to know what the single white/green wire is that goes into a yellow clip on the back of the radio is. Seems to have 12v all the time but does not affect anything from what i can tell. The radio acts the same way with or without it plugged in.