If the air coming out of the vents is hotter than the outside air temp... then it has to be coming from contact with the heater.. meaning flap /control of the flaps/ sealing of the flaps... types of things... the AC does not have any way to heat air ( to my knowledge )....
Fortunately these problems usually cost less than dealing with the AC.... but may be harder to deal with due to location and number of components potentially not working... like diaphrams, vacuum lines, temp sensors... etc....