My heater is blowing cold air in my 2002 mercury grand. The coolant is full and the two hoses going through the firewall are hot. could this be the heater core? the defrost still works and theres no liquid in the car.

Thermostate

It's ugly. If you get to the actuator, take it off and check the door by hand for smooth operation. If the door is jammed, you'll have to split the case and repair it to proper operation.
Either way, check the actuator for stripped gears or shaft key. test it with the battery to see if it works, etc. Here is the full removal procedure:

1. Before servicing the vehicle, refer to the Precautions section.
   
2. Disconnect battery negative cable from battery and properly isolate to prevent accidental reconnection.
3. Remove the instrument panel.
4. The instrument panel houses the controls for most of the electrical systems of the vehicle. The glove compartment is located in the lower RH side of the instrument panel. The LH side of the instrument panel frames the steering column and houses the instrument cluster.
   
   1. Position the wheels straight ahead.
   2. Disconnect the negative battery cable.
   3. Loosen the lug nuts on the front wheels.
   4. Apply the parking brake, block the rear wheels, then raise and safely support the front of the vehicle securely on jackstands.
   5. Remove the front wheels.
   6. Remove the retaining nut in the radiator coolant recovery reservoir assembly.
   7. Remove the retaining bolt in the front fender splash shield.
   8. Position the front fender splash shield out of the way.
   9. Lower the vehicle.
   10. Tag and remove the wiring connectors.
   11. Remove the retaining screw in the parking brake control.
   12. Remove the parking brake control.
   13. Remove the retaining screw in the instrument panel finish panel in the lower instrument panel.
   14. Remove the retaining screw in the steering column opening reinforcement.
   15. Remove the retaining screw in the steering gear housing.
   16. Remove the tilt wheel handle and shank.
   17. Remove the ignition switch.
   18. Disconnect the upper steering column shroud and remove the retaining bolts.
   19. Disconnect the ignition switch assembly.
   20. Tag and remove the electrical connector.
   21. Disconnect the vacuum hose.
   22. Remove the retaining nut in the steering column assembly.
   23. Lower the steering column tube assembly.
   24. Disconnect the shift cable assembly.
   25. Remove the steering column shroud retaining screw.
   26. Remove the retaining screws from the multi-function switch.
   27. Position the multi-function switch out of the way.
   28. Remove the retaining screw from the cowl side trim panel.
   29. Remove the lower panel door trim, remove the retaining screw, and position the panel out of the way.
   30. Tag and disconnect the wiring connectors, and remove the ground wire assembly.
   31. Remove the retaining screw on the cowl side (RH) panel.
   32. Remove the lower panel door trim, remove the retaining screw, and position the panel out of the way.
   33. Tag and disconnect the wiring connectors, and remove the ground wire assembly.
   34. Remove the retaining rivet in the passenger side of the instrument panel.
   35. Disconnect the generator.
   36. Disconnect the radio feed of the wire assembly.
   37. Unsnap the I/P finish center (LH) on the panel assembly.
   38. Remove the radio chassis.
   39. Remove the retaining screw in the instrument panel, and position the panel out of the way.
   40. Tag and disconnect the wiring connectors.
   41. Remove the retaining screw in the air bag restraining module assembly, and disconnect it.
   42. Remove and disconnect the instrument panel defroster-opening grille.
   43. Remove the instrument panel to cowl brace retaining screw.
   44. Remove the instrument panel finish panel retaining screw.
   45. remove the retaining nut on the instrument LH lower finish panel.
   46. Remove the retaining brace.
   47. Disconnect the vacuum hose on the harness assembly.
   48. If the instrument panel is being replaced, transfer all parts to the new instrument panel.
5. Disconnect the electrical connector from the blend door motor/actuator.
6. Remove the four screws holding the blend door actuator to the plenum.
7. Remove the temperature blend door actuator.

Fig. Blend door actuator electrical connector

Fig. Blend door actuator mounting to plenum

to install

1. Installation is the reversal of the removal procedure. Check air bag indicator operation

Remove the center console and on the drivers side toward the bottom of the heater box will be a small electric blend motor. Replace this motor. New part should be about 90 usd. After the center console is out should be three small bolts holding it in.

Something wrong with the flap operating mechanism