The heater blows cold air.

The door you hear is the blend air door. It is what determines the temp that enters the vehicle. check the actuator on the door. If it is moving around, you will get heat and then outside air (cool).

This guide can help us fix it

https://www.2carpros.com/articles/car-heater-not-working

I watched the door open and close.
I still don't get heat, only cold air?

Okay, I need you to check something. I thought the door was working on and off. With the engine hot, the heater on high, feel both heater core hoses at the fire wall. Are both hot?

Yes

If the hoses are both hot, then something has to be heppening with the blend door not staying positioned correctly.

Is there more than one door?
The one inside that is in front of the blower motor opens and closes switching from air to heat.

No, that is it. I tried to find a picture. The best I could find was of the actuator. Take a look and let me know if it looks the same as what you are seeing.

No. The door is vertical. Opens with Air cond and closes when I move the switch to heat.

I have the same trouble with a/c working but heat not on 2006 Grand Marquis.The blend door actuator only operates in clockwise direction meaning it will only turn the blend door to the cooling side with heater core access closed off.I removed the blend door actuator (no an easy task!)to turn the blend door lever by hand to heat and the heat works but no matter what I set the controls on dash to,the blend door actuator will not operate in counter clockwise direction to move the lever to heat.I did not test the 6 wire plug that attaches to the actuator to check for function.I'm guessing that the hot and cool adjuster on the control panel must not send a signal for heat since if the actuator is in the heat possition when I mount it,then I switch it to cool the lever moves the door to that cool direction(clockwise) but nothing will make the actuator move count clockwise to set to heat??Perhaps the heat/cool dash dial has a problem and doesn't send a impulse for heat?My dash controls are not the X type that I see most have with the code reading capability but mine has only 3 knobs with fan speed to left, middle is heat/cool blender,and right is High A/C,Low A/C,Vent,OFF,Defrost,Mix,Floor settings.All the functions for air directions to floor.mix,defrost,vents, all move and direct those duct settings correctly.These controls are below the radio.The door that the first moderator describes is next to the passenger side upper kick panel where a box has a large exposed foam door that I believe directs air draw from the cars inside to drawing from outside the car,and mine is functioning also.

when switching heat dial from cold to warm,temperature remains the same.
Water level is fine. Does NOT overheat. Cannot find vacuum leak.
I've even Replaced the appropriate fuse.
I am hoping it is not the heater core,as I know that the cost will be very expensive to repair.
I thank you for any suggestions to fix my dilemma.

your temp actuator is probably bad which is not an easy thing to replace. Apro will probably have to do this. as the dash may have to come outit's behind pass air bag/glove box, etc.

cannot get any cabin heat?

Is the coolant full? Are both heater core hoses getting hot? Is the engine warming up to a normal operating temp?

the heater takes about half hour to start giving warm air and its only warm not hot. the blower works ok its just takes a long time to warm up the inside of the car.I had the heater core changed last winter and the heater worked good then

Make sure the coolant level is full. If it is, start the engine, turn the heater on high, and when the engine gets to operating temp, feel both heater core hoses. Are they both hot?