2000 Radio speakers not working

I have a 2000 (NOT) RT Durango that all but one speaker went out. Is there a seperate amplifier that goes along with the head unit on this year. When I move the little knob that is supposed to equal out where the sound to each speaker, it acts as if there is only one speaker. For example if I move the knob to play music only on the right rear speaker, it still comes out the left front speaker(the only one working by the way). Do you think it's the head unit or amp(if installed)?