I have a 1998 Ford Crown Victoria, with 52000 miles. Last Tuesday, the heat worked fine, but as of Wednesday I get no heat even after an hour of driving.
The temperature gauge climbs to the same point it always has. The coolant was a little low and kind of brownish looking. It’s not oily, so I don’t think I have oil in the coolant/coolant in the oil. I topped off the coolant but I still don’t get any heat at all.
My initial suspicion is that the coolant has never been replaced, so I am going to have a flush and fill done. I am posting to ask for other things to check out before I have someone take the dash apart because I can’t really afford that right now.
Here’s some more info:
-The car is equipped with an electronic climate control unit on the dash, with an automatic thermostat function.
-Switching between floor/defroster/dash vents does work. I’m not sure if these are vacuum operated or not, but if they are, I don’t think there is a vacuum leak. I also don’t hear the noise usually associated with a vacuum leak.
-I have changed the cabin temperature setting with no effect.
-There is a large plastic cowling over the rear half of the engine, so getting at the heater hoses is something I’m not sure how to do. I don’t have a way to lift the car safely to check this from underneath.
If I think of anything else I will post it.
It is possible the heater core is plugged. Likely with rust, either cycled from the rad or heater core.
Yeah, that’s what I’m afraid of 
I agree. Sounds like a plugged heater core.
You may also want to check for a vacuum operated water valve in one of the heater hoses and make sure it’s not stuck.
There’s a good chance that the air temperature blend door is stuck. It is electrically activated. The other air doors are all vacuum controlled. There is an easy to do self test to determine if this is the issue. Unfortunately, I don’t recall the sequence for your car. Go to www.crownvic.net for help on this.
“You may also want to check for a vacuum operated water valve in one of the heater hoses and make sure it’s not stuck.”
No such animal on this car. Coolant circulates in the heater core all of the time. Temperature control is via an air door that is connected to an electric motor.
Wow there is a lot of information on that site. Thanks for the link 
Turns out it is the blend door motor. This looks like something I could do myself if I had somewhere to do the work. My living/parking situation kind of leaves me stuck going to a mechanic.
Don’t I get any stars for that?
But seriously, thanks for coming back to this thread and posting the resolution.
I believe that changing the blend door motor requires pulling the dashboard back. Since your cooling system has been abused by not servicing it as evidenced by the coolant color, you should seriously consider replacing the heater core while the dash is loose. Most of the cost of replacing both of these items is labor. Be sure to use a good-quality heater core.