The heat warms up and is hot on the passenger side. Defroster, vent and floor heat get hot. Drivers side blows cold air at the same time. I took to Carmax they said to replace heater core. This feels more like an actuator or vent problem. Any help you can give on what to look for would be great.

 

Sounds like a blend door acuator. Pretty common part to go bad. I usually check them for movement while switching the heat. Sometimes tapping on them gets them working again to confirm malfunction.