Black/Red wire on left harness of my '06 coupe has melted to bare metal. It's melting all the wires around it in the harness. My headlights keep turning on and off, the highbeams won't stop flickering at times, and the car horn stays for minutes at a time. I suspect the shift lock fuse has just blown, and I can no longer change gears. It's melted throughout the car, I can see the wire under the wheel and it's melted there as well.

Is there a diagram online for that specific wiring?

I have a 12v current running through all the grounds for the signal lights, the headlights, etc. according to my mechanic. I have no electrical background, so I'm in the dark here.
My mechanic has already charged me 8 hours of work, currently at $850, trying to figure out a solution here. He now claims that his only option is to replace the BSU or ECU? Could that possibly cause the shorting of that wire?

Any help is greatly appreciated! It's a shame to see my G35 in such good condition on the outside but broken on the inside.