A serious vacuum leak could cause that.
There are numerous possibilities. You really need to have the cooling system pressurized to determine what is leaking.
vacuum leak
You may just need a tuneup. Could be a number of things. Dirty fuel injectors, vacuum leak, etc.
Probably a vacuum leak
A blocked heater hose could cause a leak. A stopped up heater core could as well. Or the connector in the Windstar could be cracked.
Coolant leak? Check your heater core.
It's too full.
Oil can leak from any number of bad gaskets or loose bolts. Clean the engine at a car wash, getting as much grease off as possible. Watch closely for a few days over the entire engine, even under it, to try to find the leak.
The blend door is vacuum controlled. Any vacuum leak in the engine system can cause the door to not function properly. Check this system first. The door could be physically sticking but a vacuum leak is a more common problem.
Take it apart to determine the cause. Only then you will be able to fix it.
You probably have a vacuum leak. Check under the hood for a disconnected vacuum hose.