1999 Taurus, no heat. Not good with winter coming.

If the coolant is full and the thermostat is bringing the temperature of the engine up to par than I would bet with 95% certainty that the heater core is plugged.

You can try back flushing it. You'll need a pair of pliers, a drain bucket, a piece of heater hose the same diameter as the heater hoses (about 3/4"), and a couple of needle nose vice grips. Pinch off both of the heater hoses at least 6 inches or more from where the attach to the heater core with the needle nose vice grips. This will minimize coolant loss from the engine. Put a drain bucket under the car to catch any coolant that runs out (by the way this is going to make a huge mess). Remove the clamps and heater hoses from the heater core. install the short piece of 3/4 inch heater hose onto a garden hose. Install the other end on one of the heater core tubes. Using low water pressure flush the heater core. You should see a lot of nasty stuff come out. Repeat on the other heater core tube. Continue this process until the water runs clear.

Reinstall the heater hoses, remove needle nose vice grips, Top off the coolant in the radiator, start the vehicle and let idle. Continue to monitor the coolant level and add as necessary. You should start to get some heat with the blower setting at low. If the heat goes away at higher settings than you'll need to repeat the procedure described above. Sometimes driving for a couple of days will help loosen up the debris in the heater core.

Also if the coolant is really dirty that it will help too have the cooling system flushed as well to prevent the debris in the coolant from settling in the heater core after you have flushed the heater core.

There is always the possibility of the heater core leaking after an attempt to flush is made. Using low water pressure will help minimize this.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa