CODE 43, and 2 knock sensors?

My motor in the 93 isn't the original... It is a 4.3 Vortec W series. I need to know where the possibility of a second knock sensor may be? The one located between spark plugs on the driver's side is new. New plug too. I don't know where an additional (if that's the case) is located. Also, the book says something about a engine spark control module behind the block, but no diagrams... Is that the module in the distributor? If so, that's new too! My truck has an erratic idle. At first, code 43 showed itself at stop lights... But when I accelerated, it would disappear. Now it's constant, and idle problems... Any help? Thanks. Everything is new, sensor-wise, so I don't have a clue.

The 2nd knock sensor, which stood out like a sore thumb when I finally spotted it, is UNPLUGGED. The plug end is damaged. I'll go pick another one up today! Thanks guys.