heater has started blowing cold

Hi Guys, My hubbys MGZR160VVC or luke warm on driver side and cold on passenger side. There's no leaks whatsoever and heater does blow but only like mentioned cold or luke warm on his side and cold on mine. Could it be the doors? i dont know the proper name for these but i know they do get damaged. The doors are not allowing the warm air in the car? Also will it damage the car by not having hot air? Thanks so much. If you could tell me the name of the actual part(s) we need.