probably because its old.
take it to a garage, they'll tell ya
....check the heater control valve.and thermostat..................and the other answer is a waste of time.....................
Low coolant in the system. Defective heater control valve. Clogged heater core.
The heater valve could be failing to open for one reason or another.
When you have the heater on, does it just blow any air at all? or no air?
get a new heater switch
Check Antifreeze level..make sure there are no leaks under the vehicle.
check heater fan to see if it is working.
Check your anti-freeze levels
Likely heater core. I had same problem with 1996 S-10 with 4.3L.
It could be that you need coolant or it's low.
Have you checked the heater core? It's happened to me before.
Either a faulty thermostat or you have a low coolant level.
Low on coolant. Bad thermostat. Plugged heater core. Temp blend door inop.