One other way to fix that massive air leak is remove your Glove Box then look up behind the dash you should see that seal now go buy some foam rope from your hardware store one that's as thick as the gap.

Now the hard part fishing a two foot piece of the foam rope back behind the connection were the seal is and carefully push/pull it into the Gap replacing the bad seal.

This method is almost as hard as the first fix but not as bad but you can get mixed results the first fix depending on how good you were at taping the gap up is almost guaranteed to work.

I used this fix first and it sealed it up ok but not a 100% fix and if the gap is very wide the foam rope can fall off or get pushed inside the box this can get into the Fan and cause you problems like stopping the fan from spinning.

 

98DAKAZ, I just finished pulling my dash, found the foam seal on right side was pushed out, the one in the middle was ok the small one on left was not even there! well got everything back together & seems to be working , now my airbag light is on ?? must have hit a wire on pass side? it may have to stay on awhile, don't look forward to removing dash again so soon LOL

 

If that one foam seal falls off you can get a cold blast from outside or hot Blast depending on were you live for me it was the 115 heat of Arizona blasting in on me NOT FUN.

Took me some time to figure the problem out a shop that worked on my A/C removed the foam and never put it back giving me Hot air blasting into the cab.

 

If the top foam seal is missing its like driving with a window stuck open an inch or more that's how bad it can leak outside air.

 

Back to the OP, I think the quick test of the heater core hoses is where I'd start. Just to make sure the heater core is not part of the problem. But the mystery to me is why the engine is actually running cold. As someone noted, that could just be a thermostat issue, and it's a relatively inexpensive repair.

I'm not a mechanic, but after that I'd start thinking about whether the water pump is doing what it should. Or whether there's air in the lines. You may just need to make sure the only thing in the cooling system is coolant. My '97 had a mysterious coolant disappearing act and the truck was running fine and not overheating. My mechanic finally found a broken intake manifold bolt on the back side of the engine.

Let us know the outcome.

 

I back flushed the heater core. Poured water into the outlet hose and then blew it out with a compressor and caught the water with a jug. Had quite a bit of sediment in it, almost like sand and dirt. Flushed it twice now and still get a bit here and there. The water went from dark dark green to faded green. I cant run a hose through it because of the weather being way below freezing. But since then i've had a little heat come through enough to stay warm just not hot. Someone suggested using a pressure hose from a car wash but i'm afraid that will be too much pressure and blow out the heater core. Do you guys think this will work??

 

I wouldn't use a "pressure hose" from the car wash. Street water pressure is more than high enough. Remember, at most the system is pressurized to just over 20 psi. Your house is probably at least 50 psi at the tap.

If you can get a hose to it, moving water won't freeze. You could also use hot water, say from the drain valve on your water heater. Make sure you flush the hose out first so that there is no ice/dirt in it.

 

Most homes on "city" water have a pressure regulator on the supply line under the house. You can make up a simple pressure gauge to see how much pressure you are getting from a garden hose, then adjust the regulator on your house to temporarily get more pressure. Just don't turn it up too much or you may create a leak inside your house somewhere. My house is at 35 psi, not 50. But unregulated it is around 110 psi. Don't ask me how I know this, but it was expensive.

So for not a lot of cash you may be able to dial up the house pressure a bit, but before I did that I think I'd flush the heater core repeatedly from one direction and then the other to try and loosen up any additional crud that might be in there. When it runs clear, hook everything back up and see if it helped.

I think it is possible to flush a leak on these trucks, so I would not go too high on the pressure considering how expensive it is to get the core replaced.

 

i actually have a similar issue with my 98 dakota 5.2 4 wheel 5 speed i bought the truck it runs great heat was great but it would fluctuate 20 degrees here and there so changed the thermostat found rust everywhere and debris so i flushed the whole system now i am running cool and the heat sucks flushed the whole two more times replaced the water pump radiator and hoses now it runs even cooler and no change in the heat i am at a lose i have no idea i am gonna try the cardboard for a last ditch attempt

 

Road, have you tried flushing just the heater core?

What's the engine temp showing?