2001 deville heating problem

The heater on my deville has not been working properly..... I have tried putting coolant in I have tried to put it on different setting rear and back with the setting on my climate control and the result is the same my car won't heat up it gets to a certain point and doesn't go passed that. so i usually just put it on 85 degrees, because if i go any hotter than that it will blow cool air any suggestions? is my heater core plugged?