[shorod]

Does your Taurus have the Rear Control Unit (RCU) in the trunk? It would be mounted underneath/behind the trunk liner and will not be readily visible without removing the left side liner. If so, the wires go from the head unit in the dash to the RCU and from the RCU to the speakers. Your problem could be at/in the RCU as well.

I would suspect that you probably have a bad solder joint in the RCU for the hybrid amplifier IC for the left front speaker.

-Rod