91 PA Climate Control Trouble

My Park Avenue blows hot air from the drivers side vents and cold air from the passenger side. It doesn't matter if the passenger climate control is changed, so when it is hot out I have to close the driver side vents and get A/C from the passenger side and now that it is cold, I have to shut the passenger side adn get heat only from the drivers side.

Anyone have any ideas on what I should check?