1977 Cadillac Brougham automatic climate control problem

For some reason my automatic climate control device wont work, when I turn the car on, I dont hear the fan kicking in. The only function that works is the defrost function, everything else dosent work. Sometimes I hear a quiet rumble or a hissing noise, but no air comeing through out of any of the vents. The times I noticed it actualy working is durning the day time, but in the morning or at night it dosent work it all. How can this be fixed?