I've already done a radiator flush but I have not replaced the heater core. Would it be possible that the thermostat is not working properly and not allowing hot air to come in? Can a thermostat even prevent that from happening? I have a 1998 Dodge Ram 1500 V6 3.9 liter 2 wheel drive

 

More likely you have an air pocket in the heater core. Not the best design in the world, with the heater core being the highest point in the cooling system...... Best bet is to raise the front of the truck as high as you can, be that with a jack, or parking on a steep hill, and try and burp the cooling system a bit more. Either that, or backflush the heater core, keeping as much fluid in it as possible when you hook things back up.

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