So I recently noticed that I get no heat from my passenger vents (middle passenger and passenger). I took it in to the dealership near my house to have them check it out as I currently didn't have the time to rip it apart. They told me it was the heater core that is bad. They wanted $1100 dollars to swap it and I told them no way.

I took the car home and flushed the heater core and it was pretty clean, but afterward, I still have cool air on my passengers side vents. I am fully capable of changing out the heater core myself, but I don't see how this problem is related to the heater core. It only has one core and I get plenty of heat on my drivers side so how is the core not working? To me that seems like a door/actuator problem.

Does anyone have an information on how this problem could be the heater core and not an actuator/door problem?