First make sure the radiator fan is coming on also clean radiator coils with a garden hose and nozzle and if A.C coil is in front of radiator clean it also because if clogged with dirt and bugs will restrict air flow through radiator.
Put cardboard in front of your radiator/it helps to keep the car warm. Check and or change your radiator fluid. Check to see if you can squeeze hoses to heater core. Check for fluids on the ground.
First thing to do would be change your thermostat may be stuck open, if you havent all ready ,if this doe,s not fix the heat,next check bottom and top of small heater core hoses going into firewall,both should be warm with heater on and car warmed up,if not you have a clogged heater core.
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