[wiswind]

The heater core on the windstar has coolant flowing through it all the time.
Temperature control is done by the "blend" door that determines how much of the air will travel over the hot heater core.
You seem to know this....but want to review it for others viewing this thread, as well as keeping the focus on the heater core temperature vs air flow.

Note, that this is assuming that the fan is not significantly slowing down at low RPM......although, I would expect that if that were the case......to much extent.....then you would have other issues due to low voltage.

My windstar did not have the rear heat unit....so I am not familiar with that.
It is possible that there is air in the system that needs to be "burped" out.....
I don't know how to do this.
I have had air in the engine part of the cooling system....which causes the temperature to fluctuate way hot.....but you do not seem to be having that issue.

When the engine is COLD.....remove the radiator cap (if they still had one in the 2002).....and see if you are low on coolant in the radiator........which is possible (was on my '96).
If so....verify that the cap seal and the area in the radiator "neck" where the cap seal seats is clean (free of grit/dirt).
Then fill the radiator fully.....up to the cap...and put the cap back on.

Another thing to check....with the engine warm.......being careful to not get burnt (wear a glove).....squeeze the upper radiator hose.......is it firm (indicating that there is pressure in the system?).
If it is not firm....your problem may be that they system is not holding pressure....and you will need to find the source of that. (Cap.....on radiator or on overflow bottle, if there is not one on the radiator).

Also, verify that you have antifreeze in the water in the amount that you need .....between 50% and 70%.
Over 70% is not good......under 50% is not good.
You can get a coolant tester at most auto part stores.