How does my heater work in 1991 cadillac deville

Engine coolant runs through a heater core (mini radiator inside the dash with engine coolant in it). Air is pulled across the heater core using its heat to warm the air. The cars heater can only blow air as hot as the engine, if the engine is cold then you will have no heat.

Since its a caddy it may have a vacuum controlled valve that allows coolant to pass through the heater core. If the vacuum diaphragm has a small tear in it then it won't open fully and the car will blow cold or almost warm.