HEATER IN MERCURY SABLE ONLY BLOWS COLD AIR GOT THE AIR CONDICTION FIX NOW NO HEAT

Not in the coolant system, you flushed

It is a failed HVAC Blend Door Motor, under the dash

The fact you have heat or A/C, but not in all 5 positions
or combinations, should have told you, the air flow is not
switching as it should

this may be a goofy question BUT do you mean flushed the radiator not the heater core. Heater core is on the firewall.
Check the hoses going into the heater core to be sure they are both hot after the car has reached running temp. This will let you know if you have a blocked heater core not radiator.
Just wondering?

porbably your cabin heater (radiator like aparatus inside dash) is clogged with debris. you need to replace it if your blower works but it blows only cold air. but be sure that your engine is warm.

The ford taurus and mercury sable are very well know to have coolant pressure, heat problems. I have worked on a minimum of 300 of these specific vehicles in my career, I can tell you the system is a headache,yet manageable. Are you getting cold air through the vents? is the blower motor working? Is your antifreeze brown in color or goo like? (a tell tale sign of a broken water pump and 90% probability of a clogged heater core.
also other basics, have you flushed the system and checked to make sure the car is not air bound?

plugged heat exchanger is both heater hoses hot valve under hood closed

Have you checked for air in the system ? Some of the V-6 engines have an air bleeder on top of the intake manifold to release trapped air. What you do is fill the rad to the top , start the cold engine and open the bleeder untill fluid comes out meaning the air has been vented. With air in the system the heater does not transfer heat efficently. You can also vent an engine by squeezing the upper radiator hose with the engine off and cold. Never open a hot system. The squeeze method is easy. With cold engine remove the cap and fill the radiator almost to where it will spill over. Then slowly squeeze the upper radiator hose and you will see the air escape. When you release the prussure you will see the level drop slightly. Now if you do not have air in that system you will need to check the heater core flow. With everything closed up and full start the engine and let it warm up. Now feel the two hoses in and out of the heater core where it goes thru the firewall in one is cold and one is warm then your heater core is plugged. You can try a flush at this point but it usually needs to be replaced. Thanks for asking at Fixya.com.

Take it back. They either left a vacuum line disconnected or accidentally dislodged one.

The blend door that regulates the airflow over the heater core may be stuck in the AC position. It is inside the case and is most difficult to get to. There may be a YouTube video that may help you locate it and test it.

Sounds like the heater core is stopped up. Try removing the heater hoses and backflushing the core.If this doesn't work replace the core.

http://www.autozone.com/autozone/catalog/parts/jobDiagram.jsp?engineCyl=6&ymmeId=1999/32/13/6&imageURL=&categoryDisplayName=Climate+Control&productType=00&parentId=101

this is a helpful diagram to show where everything is....