The heater core is located on the passinger side under the dash board remove the cover screews and take it from there.Dave K. 1999 Silverado 1500 4WD 5.3 Vortex 8 owner
Short answer is you got to pull the dash, then the HVAC box (contains the core). Replace the core then remember how to put it all back. I am going to try and do it without pulling the whole truck apart, today. If it works. I will tell you how it went.
Probably heater core
4WD - 1999 Window was released on: USA: 1999
4WD - 1999 Friday was released on: USA: 1999
4WD - 1999 Bump was released on: USA: 1999
4WD - 1999 Show was released on: USA: 1999
4WD - 1999 Merge was released on: USA: 1999
4WD - 1999 Flirt was released on: USA: 1999
4WD - 1999 Gym was released on: USA: 1999
4WD - 1999 Coffee was released on: USA: 1999
18,826 lbs.