The first thing you have to do is make sure you have antifreeze circulating through you heater core. To do this turn your truck on and turn on the heater full blast,( not just the fan but your heat control valve ). Next, after 15 or 20 minutes, by that time the engine should be warm put your hand on the hoses going to your heater core. If you have circulation they will both be hot. If you do, means your heater core is not pluged and the problem is with your heat direction doors. This is usually caused from lost vacuum. You may have a vacuum hose off somewhere. Most likely, but not always under your dash, usually in some god forsaken place to get at. Start your vehicle and listen for leaks in vacuum.
Now. After checking the heater core for circulation you find that one hose is hot and the other is not that will mean the problem is in your heater core. Has there ever been any stop leak put in your radiator? This very well could be the sorce of your problem. If you are finding that one hose is hot and the other is not. Let the engine completly cool, several hours. Cold...Now remove the heater hoses from the heater core, Taking care not to make a spill (I know you will it's quite messy) antifreeze is POISENOUS to pets! Take a garden hose and flush your heater core out, both directions several times even after you get a good flow. NOTE: you will not need to use full water pressure to do this and I reccomend you don't use full water pressure because the water pressure in most cities is too strong for your cooling system components.
3rdly: If you do not have any circulation in your heater core. Both hoses are cold. Check the radiator hoses if bottom one is hot and the top one cold it means your thermostat is stuck shut and is the culprit. 3 out of 4 times this will be your problem. Usually the easiest thing to fix also.

NOW KEEP IN MIND: The first thing you must check is your coolant level. Your engine and your heater will not operate properly if this is low.

One more thing I would like to mention. You did not say how your engine heat was. You know, what did the temperture gage read. Was it hot? Was it cold after taking so long warming up and no heat coming out. If your emgine temperture is not up to normal operating temp, and you are not getting any heat. That could be because your thermostat is stuck in the open position, and the thermostat needs replacing.

I hope this helps you. If so please post and let me know.
Thanks!

1994 ram 2500 diesel heater fan won't blow air on any speed it will come on run for a short time and stop.

Have you checked the radiator fluid? Especially as the weather starts to change, this tends to be a likely suspect. It's possible it's either low, or that the water to antifreeze ratio isn't right. The ratio should be a minimum of 50/50, and no more than 75% antifreeze. If it's low, fill 'er up. If it's not low, check the ratio. You can check the antifreeze ratio with an "antifreeze tester". This is often a little tube with a bulb on one end and little coloured balls in it. Here's a link to some image results from google so you can get an idea: http://images.google.ca/images?q=antifreeze%20tester&sourceid=mozilla-search&um=1&ie=UTF-8&sa=N&tab=wi

Pretty much any parts store should have them, and it shouldn't be very expensive, but it is very good to have one on hand!

I had the sam problem in my 99 dodge ram 1500 4x4,
I replaced the thermostat and fixed the issue.
you can try it if the heater core replacement doesnt fix it for you.

This problem is because of a bar heater core replace it and you will once againg have heat