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Sounds like your heater bypass valve stopped switching. Does your engine get up to operating temperature? Or does your engine not have a temerature gauge? If you do not have a gauge it could be one of 2 problems a stuck open thermostat or the heater bypass valve.
The back or rear defrost is an electric grid that works independently of the heater. If replacing the thermostat produced heat, you would seemed to have solved the problem but the heater blower could still be at fault. If you can't detect warm air actually blowing into your Taurus the blower is a likely suspect. A faulty blower also would affect your A/C.
Check out the heater hoses for warmth. Both should be the same with the water flowing. If they are not, the water is not circulating.
There are 2 ways the heater can work. One style varies the amount of water flow to control the heat levels. The other uses the blend flaps in the ducts to push air flow around the heater core.
If the heater hoses are not circulating water, a part called a flow valve is to blame. Check out the autoparts sites for photos of the flow valve for your car. If you have the blend flap system, a solenoid called a "motor" is not changing the position of the blend flap in the dashes' ductwork.
If the heat blows hot, the defrost should also. If the heat does not blow hot air, it could be your thermostat, your heater core valve, or the heater core itself. with the vehicle being a 2002, the heater core is the least likely.
The thermostat is located in a housing on your engine that the top radiator hose connects to... but it isn't tied to your heater problem. The thermostat restricts coolant flow until the engine warms up, but doesn't control the car heater.
You may have a fuse issue if the fan isn't blowing any air. That's an easy check and fix. Autozone might check it for you.
I hope this helps... Good luck.