Speaker Problem

i could go two ways with this. poor speaker install or shorted out speaker line you should have had the original problem fixed under factory warranty first now thay are going to give you hell about the after market stuff. check and see if the speaker is properly conected, no exposed wire, good solid conection at the terminals, and that the terminals are not shorted on the sheet metal of the door. if all this checks out that you would disconect the speaker from the car and the radio get a DMM set it check for resistance,  ground one lead and probe both ends of both speaker wires  if you get any reading you have a short .if that check out move the grounded lead to one end of one of the speaker wires and probe the other two ends you should see a dead short between the mating end and nothing from the other wire now switch the  lead to the other wire and repeat. this will check for a break in the line and if the two lines are shorting totgether. if all this checks out open and close the door and repeat the testthis is incase the problem is intermentent. if all checks out i would start looking at the head unit .