The knock sensor on a 1999 Chevy Silverado/Sierra with the 5.3L Vortec engine is typically located on the
passenger side (right side) of the engine block, near the front. It's usually mounted somewhat low, near the oil filter area, but its exact position can vary slightly depending on the specific engine configuration.
To be absolutely certain, you'll need to consult a repair manual specific to your 1999 Chevy truck's 5.3L engine. A picture in the manual will show you precisely where it is. You can often find these manuals online in PDF format or at auto parts stores.
