Could be the Climate Control Unit or Clogged heater core.
http://www.markviii.org/LOD2/blend_door_repair.htm
Your heater will blow cold air only.
you may not have the heater water hoses plugged in
replace front heater corefront and rear have independent heater cores
Sounds like the Blend Door actuator, a $6.00 part but about $2,000 in labor, the dash has to be removed to do the job right.
check your coolant level
Thermostat and/or heater core may be defective
Low coolant in the system. Defective heater control valve. Clogged heater core.
Check coolant level
check thermostat, might be stuck closed.
Check for low coolant
You are low on coolant to the heater core.