On a 1999 Jeep Wrangler 6-cylinder (4.0L), the Idle Air Control (IAC) valve is typically located on the throttle body. It's a small, usually rectangular or cylindrical, electrically controlled valve mounted directly to the throttle body itself. You'll find the throttle body near the intake manifold, generally towards the front of the engine.
Because there can be slight variations depending on the exact model and build, it's best to consult a repair manual specific to your 1999 Jeep Wrangler 4.0L engine. A picture of your engine bay would also help someone confirm the location.
