The system (when working properly) doesn't just dump ambient air into the car at ambient temperature, it heats or cools it accordingly to provide the desired temperature. The problem isn't that "too much" ambient air is entering the car. If you're not getting any codes, first blow out the cabin air temp sensor with compressed air as submariner suggested. (personally I would use canned air as opposed to air from a compressor, only because it is completely clean and dry) also check that the sensors fan is running. With the car on, put your air up to the grille to the left of the steering wheel, you should hear a faint whirring sound.
If it isn't running, it might just have to be lubed with light machine oil. The fan/sensor is accessible by removing the cover on the drivers side of the dash (with the door open). Stick your finger through the rubber gromet and pull straight off. You'll see the fan/sensor is held on with one bolt (10 mm I think) remove it with a socket on an extension, carefully guide the sensor out of the opening in the side of the dash and unplug it. Hopefully that solves the problem and it's not an "in dash" sensor or blend door, they're a pain to get at.
