I am trying to get the harness out of my blazer and at the back of the engine near the distributor and it goes down and splits off into the left and right. I can not find where they lead to.

 

The part that runs down the driver side (left side) will run down to the transmission, both of the bank 1 O2 sensors, as well as the transfer case with related switches/sensors (if 4wd), and vehicle speed sensor. The passenger side should have a ground on the back of the passenger side cylinder head and run down to the Bank 2 O2 sensors it may also have some of the sensors on the back of the engine (oil pressure sending unit or switch depending on how the truck is equipped, knock sensor, etc.). I say should have a ground on the back of the cylinder head because you can tell if someone has been there before by whether they put that one back in place. It can be a REAL PAIN to reinstall.