Jimm 11/24/2012The heater control valve or heater regulator is located under the instrument panel, just next to the heater core. There are a few things that can cause this problem. Reason #1 a low coolant level can cause the heater core to receive less coolant flow hence causing the heat to come less quickly. Reason #2 sticky heater control valve, if this control valve is stuck closed coolant can't enter the heater core hense wont heat up the cabin. Reason #3 clogged heater core, over time some heater cores will clog up even sometimes start to leak in cabin. All these are normal circumstances and should be checked.