Heater Core

[Okiespaniels]

It's a PITA but can be done and usually without removing/evacuating the A/C. I did mine 2 years ago. It was time consuming and I cracked my virgin dash, had it roll backwards into my gf's ranger and do lots of damage, BUT I prevailed.
You will want to do it where your truck will not roll and can be blocked into place.
DT has a couple of good writeups on doing it.