1986 Subaru Impreza heater

Heater cores can leak or become plugged. If yours is leaking, you'll have other problems, like overheating and a puddle on the ground. To check for being plugged, feel the heater hoses once the engine is warmed up. They should be too hot to hold onto for very long. If they are not real hot, feel the upper radiator hose. If it is also not very hot, suspect the thermostat.