Temp problem with one side of car

My son's 07 chevy impala with duel temp gauges has heat on passenger side of car and no heat on driver's side. I replaced the temp actuator with no luck in fixing the problem. I also tried replacing the control panel, again with no luck. Problem still there. It does not matter if it is idling or driving, problem the same. Any ideas on what to do next without going to dealership preferably.