<!-- from fixya rte -->Why is there steam coming from my defrost - 1998 Ford Expedition

Most likely the "steam" is vapor from a coolant leak. Does the steam have a sweet smell? Does it leave an oily residue on the windshield when using the defroster? If so, you probably have a leak in the heater core.

if the "steam" leaves an oil film on the glass, your heater core may be leaking coolant.

sounds like you have a coolant leak from the heater core or a hose under the hood leaking you need help from a mechanic

Very possible the heater core may be leaking, causing the steam to come through the defroster and dash.

Be careful with that Barsleak. Don't be surprised if you see seals and or your radiator blown out. Not only will it plug the leak but it can also plug things that you do not want plugged, such as hoses and seals

Sounds like the heater core is leaking coolant. If it is leaking it will blow steam out of defrost vents and fog the windshield. Symptoms: low coolant level in radiator, steam coming out vents when engine is warmed up, odor of anti-freeze coming out of vents, wet carpet in floorboard near heater blower motor.
If so, replace heater core.

if it is sweet smelling steam, it is a leaking heater core. They are hard to get to. If a small leak, add a can or bottle of stop leak to the radiator or coolant reservoir.

The no air flow to the defrost ducts is confusing since, by law, the system is desgined to fail to defrost if there is a system problem, like a vacuum leak to the heater controls.

Sounds like you have 2 issues. Is the coolant low in the radiator and overflow bottle?

report back.

Should be a central vaccuum block behind your selector for vent defrost etc. perhaps that is going bad. They're expensive for something made of plastic. cheaper alternative is the junk yard.

Your heater core has corroded through. You need a new heater core. You may be able to get by temporarily, with bypassing the heater core hoses, located in the engine compartment. These hoses carry coolant heated from the engine, to the heater core. You remove the hoses from the core, and put a pipe in the middle, connecting the two hoses together using hose clamps.

Should there be a cool morning, or if you need the defroster, you won't have warm air. Just a temporary fix.

You heater core is leaking causing steam to come out of the interior vents. This is quite a process to replace a heater core in one of these cars. The entire dash needs to be removed. The core itself is located under the middle of the dash towards the top. If you do this job yourself, take your time and allow a full weekend to do if you've never done one before.