1978 Isuzu Rodeo Question Heat going cold

First, with the engine cold, look inside the radiator to see which way the coolant passages are facing. They will either be a cross flow or vertical, most are cross flow. You may not be able to see them though. If the radiator is not full, top it off and start the car with the radiator cap off. Let the car run just off idle while watching the coolant level. If it starts to over flow put the cap on and feel the upper radiator hose. It should be cool until the themostat opens and the hot coolant leaves the engine to get to the radiator. When it does you can take the radiator cap off very carefully, with a rag over it to help prevent getting burns. Squeeze the upper hose first, if you can't squeeze it there is pressure in it so extra care is a must when loosening the cap. Slowly rocking the cap while loosening will help relieve the pressure. The coolant level should have dropped and you should add coolant. Alot of times if the coolant level is not completly full it doesn't carry enough heat to the heater core. If the upper hose never gets hot then my guess would be a stuck open or missing thermostat.