roughpaint

hi. i've got a 94 windom, great car, bullitproof. I have lost sound to my left rear speaker. checked the speaker and it is fine, just not getting a signal to it. have the haed unit out and seats and amp, but cant find a wiring daigram or anything to help me trace the problem back through the amp and to head unit so i can see where the signal stops (if there even is one). has anyone seen a problem like this or have a full wiring colour code?!?! thanks. help!