I'm Chris, Thanks for coming to Just Answer, I am going to do my very best to assist you with this concern in a timely matter. If there is any doubt or further explanation needed on my answer, just reply on this thread and I will get back with you. I am here to help you, so I want to make sure your satisfied.
Step one is getting the vehicle up to operating temp and grabbing both heater core hoses in the engine compartment on the firewall while heat is being commanded. Both hoses should be extremely hot meaning there is good coolant flow. If there or one is cold then you have heater core blockage which a flush may resolve and if not then a new heater core is needed.
If the hoses are hot then lets check for a broken blend door or a broken blend door cable. Remove the cable connection and try to operate the door manually to get heat.
I have attached some pictures and files showing pictures to aid in access and diag.
could there be air in the cooling system and if there is how do i bleed