Is heater control on Auto? Are you set to Recirculate? Defrost mode always sets fan to higher speed.
Personally I don't use the Auto fan control, I hate having it blow full speed when I start on a cold day waiting for the engine to warm up. And I leave the auto temp off and adjust temp up or down as I like.
If AC is not working then there are other things to check.
First, we need to know if it's a standard climate control system or a nav system, since the HVAC systems are totally different between the two.
That said, really the most likely thing it could be is a bad fan switch. Power is supplied to the fan all the time and the switch provides the ground either through a resistor for slow speeds or directly for high speed.