Air Mix/Mode Actuator (Blend Door)
1999-00 models use cables to actuate the temperature door and re-circulation doors.
The blend door is a servo responsible for controlling the mix of warm air from the heater core and cool air from the A/C and outside.
See this http://www.autozone.com/.../...rGuideContent.jsp
Insufficient coolant , trapped air in the cooling system might not have enough coolant to full fill the heater core.
The hot coolant flows from the engine to the heater core through the "inlet" hose. After circulating through the core, the coolant returns to the water pump through the "outlet" or "return" hose. Both the inlet and outlet return hoses should feel hot when the engine is at normal operating temperature and the heater is on.
1. If the both heater core hoses were cooled, then this vehicle might have an insufficient coolant in the cooling system.
2. If the heater core inlet hose is hot and the outlet hose is cool, the heater core might be clogged.
Note: check the radiator coolant level and top it off as needed.