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Have you check the coolant temp in the upper tank of the radiator is it 180+ when warmed up?
Are the coolant hoses going to the firewall and into the heater core the same temp on both and about the same temp as the other hoses?
Do you have Auto A/C or manual controls?
Do you have any history on the car leading up to this problem?
Let me know what you find out about the temperature readings in the radiator and at the inlet and outlet tubes of the heater core. If the heater core is restricted one tube will be substantially colder than the other two going into the heater core. In order to replace this heater core the entire dashboard needs to be removed and the heater box disassembled to extract the heater core. At this point you may want to consider having a flush and back flush the heater core several times to see if you can get any debris to come out of the heater core. If your temperature gauge in the combination meter is rising to the middle point of the gauge where normally does I would agree that the thermostat is not the issue. If the temperature gauges down near cold constantly than the thermostat could be stuck open. If the thermostat was sticking close to you would have tons of heat and the vehicle would have a tendency to overheat. Make sure that when you slide the hot control lever you hear a distinct thud or movement of the heater control bypass doors. Let me know what you find.
The actuators or air blend door is mechanical with the cable or depending on your model may have an electric servo. There are no vacuum lines going to any actuators. Based on your description it sounds like this air blend door which controls the amount of air that passes through the heater core or around the heater core to supply the temperature that you have it set at.
If you have manual controls for your air conditioning there are three control cables one that goes out to the water control valve one that goes over to the inlet air door by the blower motor, which allows fresh air to enter the vehicle, and one cable that goes down to the air blend door. If you look up under the passenger side towards the center you should be able to see the cable that controls the air blend door it is on the bottom side of the air box in the middle of the vehicle. As you slide the heat control lever you should be able to see this cable move the door mechanism and lever.