1998 Dodge Truck No Heat

I have a 98 Dodge 1/2 ton, we replaced the thermostat in last winter because it wasn't blowing heat (just cold air), flushed the system and still nothing. We checked the heater core lines and their not hot but warm after running for a half hour. Put card board behind the radiator and still no heat! The only other thing that I have research is maybe the water pump? What do you think? It's -20 below and the only thing I have to drive, and with kids this is really hurting us.