2002 Mercury Sable cool heat

What you need to check:
-your level of coolant; make sure it's not low. If low add coolant.
-your thermostat could be stuck open; is your car taking a very long time to get to normal operating temp?
-your heater core: with engine on at normal temp and heat on high, both inlet and outlet hoses to the heater core should be very hot. If one is not, or significantly colder than the other, your heater core is probably clogged. Flush it or replace it.
-your blend door actuator or the blend door is defective. Broken blend door is a common problem for your car. Can you hear the door moving when you turn the switch from heat to cold or vice versa?

Hope this helps to diagnose your problem.