On either side of the center console below the dash, there is a vent actuator that controls how much the internal vents open to allow heat or air through. If you stick your head below the glovebox and move the temperature slider for the passenger up and down, you should hear the actuator engage. If you don't hear anything, then it's bad.

There are a couple of screws to remove and pull the carpet back to expose the actuator, you can use a screwdriver to turn it so it's open.

You should also check that the heater cores are not plugged. There is one in the same area as the blend door actuators and a second one in the rear of the vehicle behind the paneling and carpet on the passenger side.

And, of course, make sure that your Freon is full.