Possible Causes

WARNING: This page is about a different car, the 2002 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.

Central Junction Box (CJB) fuse No. 58 (10-amp).
Central Junction Box (CJB) fuse No. 36 (7.5-amp).
Heater core.
EATC module.
Blend door actuator.
Actuator mechanism.
Open/Short circuit.