For some reason the knock sensor on this truck is just laying in the engine bay on the passenger side ontop of the intake manifold, no clue why, i cant seem to find where it bolts in at. Its on a '99 Trailblazer 4DR 4WD with a 2000s block out of a similar configuration Blazer.

 

Mods
I don't 100% know if this will apply to your year but my Morf was a 92 and this what I have.
Depending on the threads on the knock sensor some were installed in the head into the water jacket - some were in the back of the head and just screwed into the head mine has both .....

 

That's because the knock sensor mounting hole in the block changed. Early sensors (pre-2001) were 1-wire. Later sensors were 2-wire and completely different with a different size mounting hole. You should be able to find an adapter made specifically to mount the old style knock sensor on the 2001+ block. And no, you can't use the 2-wire sensor in your 1999. These sensors mount vertically next to the distributor.

early 1 wire sensor is 1/4" NPT thread
2 wire sensor is supposed to be 5/16-18 thread

 

Its a Pre 01 Block, early 2000 casting.

 

then look for the mounting hole close to the oil pressure sending unit.

 

Im not quite sure where that is either les.

 

oil pressure sending unit is beside the distributor

 

Maybe these attachments will help - 1999 and 2000 have identical sensors in identical locations. 2001 uses different sensor, same location, with different size threaded hole in the block..

 

Located it, its right where the 99-00 Manual you linked says it is but its definitely too small for the one-wire type that was hanging on the harness, Which is odd because the receipt for the block when it was swapped several years ago says it came out of a 2000 4x4 4WDr Blazer. I dont know what the hell is going on here.

 

Maybe it wasn't original to the Blazer it came out of. Obviously engine is a 2001+. How about engine ID code on block at front of passenger side head