When I purchased my 5.3 for my OBS Silverado LS Swap, I failed to notice that a portion of the wiring harness was cut off and missing. So the other day I went to the local junk yard and retrieved the missing piece of the wiring harness which was for the MAF sensor and the passenger side O2 sensors.
In this video, I sit with my multimeter and trace and label all of the unknown wires. It is extremely important to accurately identify and label each wire because if you get wires mixed up and not correctly connected then you will be pulling your hair out trying to figure out why your project is not working or giving you lots of electrical troubles.
This is why it is Very Important to get a Complete Wiring Harness with your project. If your harness is all butchered up then you may be better off finding a complete whole harness or just buying one online already done. I am doing my own harness in order to save money so my experience may not be typical with yours.