1999 Dodge Ram 1500. Heater blows cold air. Checked all doors and are working OK. Have never had good hot air since owned truck 6yrs.

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Reverse flush the heater core, using a commercial radiator/heater core flush product or CLR. Personally, I would reverse flush the whole cooling and heating system, remove both drain plugs on the engine block, and refill with a 50/50 mixture of antifreeze and distilled water. Be sure to read your Owner's Manual on how to refill the system, to purge all air from the system.

Is there a thermostat in the Truck?Temp gauge reading normal?

Check the cowl for leaves and whatever else that could clog up the heater core
Sometimes you can take a flexable claw tool to get a sample of what's down in there

Feel the heater hoses and see if they are both hot(heater on full and engine warmed up)