I'm not familiar with this model so I'll go by the usual culprits. My guess is that the air duct to the passenger side has had its operating mechanism come off. This will entail removing the under dash kick trim and a certain amount of gymnastics to lie in the footwell while someone operates the heater controls, You should be able to see(with a flashlight) the control cables etc moving. It may be just a matter of clipping a cable or cable end back onto the flaps. If you turn the blower on you may find a part of the trunking displaced, by the rush of air into the dash and not down the vents.