Battery Wiring / Main Engine Harness

Hello all,
I have a dead 06 Tundra DC SR5. 140k 4x4

Looking for a diagram or some knowledge on where to start with a "bad" main engine harness and snipped battery wiring. I'll explain

Driving. Check engine light comes on (secondary air stuck). I bring my truck into Toyota to look at this, and a parasitic draw I've had. (Also, radio cut in and out sometimes).

An $800 bill and a non-running truck is what I walked away with (long frustrating story). They said main engine harness. After Toyota, lights would turn on but shut off when the key was turned, didn't turn over. I don't think the starter clicked.
Quoted: Parts = $4,000 / Labor = $3,500

I bought a used harness and it sits right now.
They couldn't tell me what part of the harness was faulty. Just a bad harness..?

*The previous owner had some janky terminal hardware to accommodate a kill switch into the cab to help with that parasitic draw. So, I snipped off the ends of everything connected to the battery and pulled out the kill switch. Right now, it sits with a potentially faulty harness, and cut battery terminals, hardware saved. If I have to replace, I want to while I'm in there for secondary air light. (Harness Pictured) Thanks for the help!

 

I would suggest posting this in the regular first gen section and making a new thread if you want to get more input on this.

 

Thank you sir!

Will do