Location of the blend door actuator

Usually, this problem is caused by a vacuum leak in either the vacuum reservoir or in the line going from the reservoir to the actuator. If the problem is that it is just not warming up, then I would first make sure that the engine is warming up like it is supposed to. If the engine never warms up, then the heat will not work. Change the thermostat if that is the problem. If that is not the problem, then flush the heater core. Sometimes they collect gunk over the years and that gunk eventually clogs up the core, keeping it from getting the hot coolant that it needs in order to heat the air being pushed through it by the blower. Basically, you would pull the heater core hoses off (at their connections to the engine). Then connect your water hose to one hose, and point the other hose away from and electrical stuff. And then you turn your water on to low pressure and it will push the gunk out. Once the water starts flowing out of the other end of the core, turn the pressure up a bit and let it run for at least five minutes. Then put the hoses back on and try your heat. Then later on, after the engine cools all the way down, change your coolant, because it is old and creating that gunk that caused your problem.

Check out this guide which can help and the temperature actuator in the diagrams below.

https://www.2carpros.com/articles/car-heater-not-working

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.