Non-mopar tech question, sorry...

I posted this on the fortrucksonly.com site yesterday, but haven't gotten any responses. I know the member traffic here is larger, so I figured I'd post it here too.

My 92 Ranger, Sam, doesn't have any heat in her at all. I swapped the thermostat because I already had one, and I have some heat now, but it is intermittent. It will get warm while the truck sits and idles, but as soon as I start driving the heat cools off. Does this sound like a heater core issue? Ford heat is notorious for melting you out of the truck pretty quick, but Sam takes forever to put out even the little heat that she does. I'm leaning toward the heater core, but it's not leaking, the windshield isn't fogging up, and I don't smell antifreeze. Of course, those symptoms all point toward a leaking heater core and not one that's just gunked up inside. I would appreciate any and all suggestions. Thanks in advance!