If it is defective it will need to be replaced.
mercury villager heater core 2001
On the bottom of the heater/AC housing under the dash.
Remove the glove box. The front blower motor and the resistor can be accessed from there.
the only goofie looking board you will find under there
Inside the dashboard at the right.
I would not recommend that you try. A block heater is preferred.
When the heater resistor goes, your climate control fans will only work on the fourth setting. You can get a new resistor from any Ford Lincoln Mercury dealer, though they may have to order one.
There would only be a rear heater core if there was a rear heater, which I don't believe was an option.
Near the firewall behind the engine, or under the dash near the heater core.
In the lower part of the dashboard on the passenger side.
I have a 95 villager and it came from the factory with a block heater.Look around the headlights for the plug tucked away.
the heater blower resistor is located on the passenger side footwell near the firewall. a complete how-to replace the resistor, along with photos is available on newcougar.org http://www.newcougar.org/forums/how-tos/3732-fixing-heater-blower-resistor-install-photos.html