Most likely a bad heater core
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
How much will it cost me to replace the heater core in my 1999 Mercury
The heater blower motor on a 1999 Subaru Impreza is in the engine compartment on the passenger side of the vehicle. It is located on the firewall.
The Heater/Blower Relay is on the blower motor housing under the dash in the passenger compartment on 1999-2002 models
You'll find the heater blower motor resistor in the heater/air conditioner compartment. This compartment is location underneath the dashboard. This is the part that allows the heater blower motor to change the fan's speed.
You see that knob you turn to run the blower? That's it.
Check the blower resister
It should be in the heater blower motor case which is located under the glovebox.
Right side
Inside the dashboard at the right.
You can test your heater blower motor by connecting a ground wire to the ground. Connect a power wire, from the battery to the positive post. If the blower motor turns it is working, and if it does not it is bad.
On the blower housing. Remove the glove box to gain access.