Stevie Palmer

So heater/ac works on lowest setting blows out cold. Any other setting blows out warm. All ducts open and close. I can do cold on drivers side and heat on passenger. When it's cold it's ice cold. When any heat setting it's like fire even at like 68. All the fans work all ducting is clear. No weird smells. It seems like rear passenger vents don't really blow out much not sure if that s related. Also just did a full coolant flush and replace. Problem was around before then was just hoping it might have a made a difference (it didnt) also about a month ago replaced the blower resistor in the glove box blower thinking that had failed. It's bizarre I'm thinking maybe air trapped in heater core? Any thoughts ?