I was raised under the hood of a truck just so we know where we are at . i cleaned out the block radiator heater core new thermostat and water pump. coolant acting as it should but no heat cold air when driving, if it warms up and i shut off for five minutes turn back on app. twenty seconds of heat flows then goes back cold..car is well maitained, an old boy at another parts store said the module for the heater is bad,,,refrenced a door under dash if not working properly will give this syptom..any ideas