1996 Chevrolet impala

I have a feeling the heater core is clogged. You can try backflushing it but given the age of it may just want to go and replace it, and when doing it also would want to do a flush and fill of the whole radiator fluid system to get all the old coolant and debris out of the system. Also while you are in there may want to replace the flow restrictor and the T fitting that goes between the heater hoses and the coolant tank as both parts are plastic and known weak points in your car.

For just pumping out HOT air all the time with no adjustment, it could be the Heater Water Flow Control Valve also known as Inline Restrictor Valve, or the HVAC Blend Door Actuator, or the blend door itself is damaged. May want to take it in to have it looked at.