It sounds like a bad core to me too. If you don't have a bad heater core then does your A/C compressor come on when you put the heater control in the window defrost position? The A/C is supposed to work in defrost position to remove moisture from the windows and inside the cab. The A/C in conjunction with heat cleans the windows off.
It could be low on freon, bad comp., bad clutch, bad selector switch, etc. Your going to have to do some trouble shooting to find the problem.