I would think the most likely cause would be a faulty mode door actuator that controls the air to the passenger side. There should be a self-diagnostic test for the climate control system listed in the factory service manual. Going through the self-diagnostic test would likely be helpful in confirming the faulty actuator. The bad news is that if it is the problem, the dash will likely need to be removed in order to access the actuator. The actuator itself is usually around $35 +/-, but the labor is where the expense lies.