Heat on Cold air out on 2002 Mercury Sable

My car is a 2002 Mercury Sable GS, The past year every time I turn my heat on all that blows out is cold air. I have had a coolant flush, change the thermostat and the heating core and yet all it does is still blow out cold air when I turn the heat on. Why?

Asked by for the 2002 Mercury Sable
I think you have a problem with your blend door.
A blend door swicthes or (flip) from side to side in your ac and heating system.
Distributing either hot or cold air. Sometime it's the blend door or the vaccum that
operate it.(under dashborad)

My 2000 does the same thing. Where do I look to check the condition of the vacume lines; under hood? under dash? If under dash what location specifically.
1 more answer
clean out heat pump and change coolant