Like someone else said, without proper diagnosis you end up throwing parts at it and wasting money.
On mine I had the same symptoms and first I checked to see if the control module was asking for the right temperature, with a sprinter capable scanner. All was good there.
Next was to check the diverter valve. It is very easy to check. It has an electrical connector with 2 wires. What you need to do is to unplug the connector.
With nothing plugged in you should get hot air out of the vents, and connecting 12 volts to the plug should give you cold air. You don't need to worry about polarity. It might be that it is the other way around, nothing connected is cold and 12 v connected is hot, but I believe it is the other way around. Nontheless, if you get cold both ways, and you checked that the control module asks for the correct temperature, replace the valve.
After replacing mine I opened it up and it was mechanically busted inside and wasn't flowing hot coolant to the heater core.
Good luck.
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