make sure the engine is getting up to normal temperature. You may have a stuck thermostat.
Feel the heater hoses where they go through the firewall. After the engine is warmed up, both hoses should be too hot to hold on to. if both hoses are not hot, there are a few possible reasons.
- the heater core is restricted
- there is air trapped in the cooling system (low coolant or leaky head gasket)
- bad valve in the hose going to the heater core.