The IAC (Idle Air Control) valve on a 1999 Chevy Tahoe is located on the throttle body. It's usually a small, cylindrical component mounted directly to the throttle body itself, often near the top or side. You'll likely need to remove some air intake components to see it clearly.
A picture of your specific Tahoe's engine bay would help provide a more precise location, but that's the general area to look.
