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.

change the thermostat, Put a higher numbered one in it. Like a 190 or 195. It's possible someone put a cooler thermostat in it before, or it's possible the current thermostat is messed up.
It might not fix it, but it's worth a try since they are only 5 to 10 bucks with a new gasket. Don't forget to get the air bubbles out again once you put a new thermostat in it.

My 96 Ram 2500 with a V-10 takes forever to warm up. You would think with all of the cylinders moving it would build up heat quickly. I think the rad & coolant system is so large & effecient it never wants to warm up. It has never had the greatest heat. I have changed water pumps, t-shats to the higher 195, spent the $80.00 on the OEM t-stat also as recomended by everyone on here, etc. Nothing works. Every year or two I flush the heater core & it gets much, much better with heat in the cabin but still takes forever to warm up. Been this way since 96.

first I'd shoot the stat housing/rad with the IR gun & see if it's getting hot enough then I'd see if there's enough flow thru the heater core & if (1) & (2) are good then see what vac door/servo is inop/cable misadjusted

I can not say what brand of thermostat was in a 1999 5.2, but the factory thermostat in a 1995 5.9V8 was a Robertshaw with a steel upper body and brass lower arms.

The lower brass arms cracked,
distorting the mechanism so that
it could neither fully close or fully open.

But the engine would not overheat.

There is a Stant 205 degree thermostat that makes a feelable difference in heater output.

Does the motor get to0 a good temp?Before you change a stat thats the first thing.Then if its right feel the heater hoses.They should be close in temp.If ones is colder than the other iots the core.Try flushing and reverse flushing it.My 99 Durango had just warm heat.I flushed it and the heat was better.It probally needs a core but Its a pain to change.Last one I did at the shop was about 4 to 5 hours.I run the 195 stat in it.Rocky

The heater cores are problematic on these, I did a pile of them while at the dealership and I'm doing my '98 Ram tomorrow (along with the evaporator)

The cores are aluminum and are known to be a problem, a 8-9 hour job I'm not looking forward too. Heat has been slack the last few years and now the dreaded steam is rolling out the vents...

If the temp gauge is reading normal and the t-stat is ok it could be the blend air control but they rarely fail, can't hurt to check that the door is in the full heat position.

...Yes ...the dreaded steam from the vents.....I'd be awfully afraid if the OP flushes out his heater core. It's pretty chancy.....
Just isolate the core as the problem, and then all you have to do is put a whole day aside to change it.

Wanted to update this thread. I just got done flushing the heater core with a garden hose- both ways. Then I filled the heater core with coolant through the heater hoses to make sure there were no bubbles. I now have heat!

Oddly enough, I didn't see much come out when I flushed the core. I had it running into a bucket.

Anyway, next week we're headed for single digits and this is going to be much better.

Success! That's good to hear especially with cool(er) weather coming on. Actually heater cores do not flow as much coolant as you'd think as it is only a mini parallel bypass & most of the psi/flow is the main cooling system so you do need em to be unobstructed