My 92 chevey cavalier is overheating the heater doesnt work and i already replaced the thermostat

often the heater core or radiator gets plugged and doesnt flow good, try taking off heater core hoses going in to the motor and run ur garden hose as high pressure as it allows thru one side and let it flow thru heater core and watch the water out other hose, if its ugly or slow moving at first ur gonna fix the problem.. same with radiator

Also make sure you have no trapped air in the system that is stopping/slowing the coolant flow.