1997 audi s6 c4 /rear speaker problem

[aky]

hi i'm a newby,i have a s6 2.2 20v turbo and since i've had it the rear large speakers don't work but the seperate tweeters do,which i find strange as they plug into the main speakers,it still has the original head unit and cd changer in it if that helps at all,any help would be appreciative as it's dooing mi head in lol.