I don't think it's leaking, you would smell it, as Mr. S says. I read your original post early this morning and I used my last 5 minutes before work to look up heater core access in the factory books. It didn't say much! You go under the dash on the passenger side, and there are apparently 5 screws, and there it is. The book refers to a special tool to disconnect the core lines. I hate it when they say that.
If it was the heater core, the tools you need are a 7-8mm socket for the 4-5 screws on the box directly above the pass floorboard. A flathead screwdriver to undo the heater hoses. Take cover off inside the truck, undo heater hoses, slide core back....then install new one that costs $25 or less.
I replaced mine in dec when I got my truck. The "book" says to drain and refill the radiator, but the day previous the water pump was changed and fresh coolant put in, so I avoided that step.
I hope all is well and you're back in business. Just giving the 411 on the heater core replacement since I've done it.