First sensor in the exhaust if going down-tube from the Turbo, 2007 6.7L, where/what does it plug into? The harness coming across the transmission, or engine harness?

Back story. I just bought a 2007 with a 6.7L, which I have never worked on before, with engine damage (threw a rod through the block). I just had an engine swapped with a junk yard long block. The guys who did the swap can't figure out where to plug it in. I took a look and I can't find any un-used pigtails. Everything else it plugged in.
My truck is throwing codes of coarse and dinging at me like crazy. Any help is appreciated.

 

I believe that is the first O2 sensor and plugs into the main harness.