There are two little motors that control the airflow on the HVAC unit. Your problem could be many things... the doors themselves get sticky after a while and don't want to move. Maybe the motor is unable to move them? This can happen if a car has been sitting for a while, or if the a/c has not been used in a long time. If the motors draw too much current they can cause a thermister to blow on the control unit.
Of the mode door actuators, one of them is easy to get to, the other not so much. They are available from the Fiero Store for $20.00
If it were me I would first replace the head unit with a known good one and see what happens. If that doesn't fix it I would buy two actuators and replace them and see what happens. If that doesn't fix it you might have a wiring issue somewhere in the dash between the two.
[This message has been edited by jscott1 (edited 05-31-2015).]