We need to check the blend door actuator which is what controls the air being directed over the heater core. This is what is needed in order to get the air hot. Your actuators are run by vacuum lines so if this is cracked or disconnected then the actuator will not move the door position.
Take a look at these attachments and let me know if you have questions. Thanks.
Images (Click to enlarge)
Friday, February 7th, 2020 AT 5:19 PM