somtimes a mouse moves into the blower motor and makes a nest ?
it could be the heater is leaking!
Sounds like you have the beginnings of an a/c vacuum leak. Our Expedition did the same thing and eventually it defaulted to blowing out of only the heater vents, which is the default when things aren't routing properly.
The heater may not be on - but the fan still blows air through the vents when they're open !
bad heater core bad heater core
The heater blower may not be working properly. Without the heater blower working, the air will not come through the vents.
It is more then likely your heater core. is it fogging up in your vehicle?
Check the heater core, it is probably bad. It could be heater hose connections to the heater core, but I would bet on the heater core going bad.
The steam could be coolant from a bad heater core.
the heater core may be plugged up
Leaking heater core
Means that your heater core is leaking coolant. Has to be replaced.
If the vents are inside the cab of the truck, this could mean you have blown your heater core.