To revive this thread, I'm in a very similar situation. No heat in my Ram, and haven't had heat really for about 2 years now. Just never had the $ to fix it, and last year it sat on my driveway while I was away at college. Now I have it with me at college and can't deal with the cold any more. We had it looked at when it first went out, but I don't remember what they said it was, and they weren't certain about anything, it was their best judgement. My truck is doing the same thing as the OP, when it's turned to full heat, and I turn the fan on, it blows hot air for about 5-10 seconds and then gradually gets cooler until it reaches ambient/cool temp. The thing is, I can do this multiple times while driving. For example, I can do it once, and then maybe 10 minutes later I can do it again. And repeat...
Regarding thermostat, radiator flush: This past spring the truck overheated on me and we had the radiator flushed and the thermostat replaced, and still no heat. Would the radiator flush have also cleared out the heater core?
When the air blows, I don't smell any antifreeze through the vents, so I guess that means no leaking core, right? Also, today I listened for the actuator (I think?), and as I turned the temp control from I heard something behind the cupholders, I guess it was the blend doors moving? So a mode door isn't the problem? Although I do notice the air comes out noticeably stronger some vents than others, especially on the driver side vents vs passenger side. But that's a different mode door right?
I'm thinking it's a heater core problem. If it were a blend door problem, I wouldn't be getting that first 5-10 seconds of hot air, right? So does this sound indicative of a clogged heater core or a broken one? Is there any other way to check?
Thanks!