'93 Cutlass Supreme Heater Issue. (Its getting cold fast)

Alrighty then. My A/C works wonderfully. Its cold, it blows and all of the switches work. Everything is basically soild. My heater worked just as well last winter, but for some reason it has decided to take a dive and I can feel the cold creeping up on me worse every single day. Air comes out all of the vents, but it's cold. Tried replacing the thermostat, and I don't think it could be the core as I've had no leakage in the passenger side. The Thermostat we installed is a 180*. The car's coolant is fine and it seems we are getting flow to every part of the car that needs it. My father, a pretty seasoned mechanic himself, is as stumped as me on this thing and we need some help!