[mprophet]

Thermostat is most definetely your problem i think its just a matter of 3 bolts and the houseing is apart, pull out the old thermostat and see if its in the closed position if its not that definetly your problem.If you want to further check it put the thermostat in a pot of water on the stove and bring it to a boil you should see the spring retract and the thermostat open up. If the thermostats the problem drop in a new one i think there about 5 bucks. There probably will be a few temperature types of thermostat when you go to buy one , if the car hasnt had a problem over heating then get a high temp one and it wont open to soon so your coolant temp can get higher thus your heater core will get higher.

If all of the these are working properly, blower motor,thermostat, and heater coil, then ther is one other possible thing that could cause this and i havent been under the dash to see how its hooked up. The switch that switches from cold to hot on your dash is most likely a mechanical switch that simply slides a door over a vent to either allow or not allow air to travel one way or another. If that switch has a broken link of some sort it wont flip the door so the hot air pases thru the heater core instead it would be blowing by the ac coils which wouldnt be on so you wouldnt get any heat and if you turned the air on to outside venting you would only get air from the engine compartment. I may have lost you on that but picture the front door on a house in the winter and you have the heat on in the house, if you open that door the heat tries to go out and the cold comes in but if you leave the door shut then the heat stays in and the cold stays out, basically the slidder that adjust hot to cold air is opening and closing that door, if the door is stuck shut then you never get the heat. Its a long shot but a possibility.