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Check the heater hose temperature on both heater hoses as close as possible to the heater core. If one hose is significantly colder than the other then you might have a plugged heater core.
This engine also has a bypass built into the thermostat housing, if it is damaged it will let coolant bypass the heater core giving you poor or no heat.
You can try to see if you can hear or feel the blend door module moving by pulling down the glove box so that you can see behind it and then turning the switch from hot to cold. You should be able to hear clicks and noises from the blend door if it is moving.
You could also pull the blend door motor and then plug the electrical connector in and see if it moves when you turn the switch from hot to cold.
Do you need diagrams for replacing the blend door?
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