I think I am having the same problem with my Jeep Cherokee Sport. I just get cold air when I turn the heat on. I think the problem may be that line going into the heater core may be clogged.
I have been turning on the engine and then squeezing the two heater hoses one at a time as fast and as hard as i can to try to move stuff around in there and it has been kinda working to get heat for at least a day or two before it clogs up again.