Okay, I have a 99 Ram 318 and the heat has never been very good. Here's what I've done to resolve the problem so far:
Get air bubbles out by parking truck on a hill and filling radiator with truck running.
Antifreeze level is good.
Just replaced the blend door coupler because I heard this was a common problem, but original coupler was fine. Not cracked, etc.
Truck doesn't overheat, so I'm pretty sure the thermostat is good. It was replaced about a year ago anyway.
The only other thing it could be is a plugged heater core, correct? Is there anything else to check?
Thanks in advance.