It could be the connections at the speakers. Pop off the speaker grill, remove the mounting screws and pull the speaker out. Check to see if both speaker wires are connected. (I had a dead speaker which had one wire disconnected.) If they are connected, pull each wire spade connector on and off a few times to wipe the connections clean. Also be sure the connectors are tight. If they aren't, sqeeze them a little with a pliers. If this doesn't do the trick, try substituting another speaker while you have the wires out. If all this doesn't work, you may have broken wires where they go into the door or the connections at the head unit may be loose.