Something is not right here. I have been rebuilding all my 69 harnesses, taillight to headlight, so I have been "into" the inner workings of these lately.
First, the factory wrapped the engine harnesses in non-adhesive tape. If the tape is gooey "electrical" tape, then it is an later add-on.
The firewall harness is seperate from the headlight harness. There should be NO wires that go from "behind the headlight" to the starter from the factory. Both of those harnesses go into the fuse panel seperately. The path of electricity does jump from Batt harness to starter harness, but it goes through the ignition switches under the dash first.
If a wire goes up in smoke, then you have a short, plain and simple. A short can also be a wire that had insulation exposed to a 12v of a higher amp rating that then runs down the wire and exceeds is rated capacity and the insulation cooks. Finding the short is the fun part.
These 40 year old harnesses with previous owner's "improvements" are dangerous. Either buy new factory replacements or you have to take off the tape wrap of the harness and inspect them.
That is what I have been doing. Every harness (taillight, middle, dash, engine, headlight) has been removed, stripped of wrapping, visually inspected for char (found a lot!), non factory splices, connector damage. Then using my spare harnesses, repairing, replacing, re-splicing correctly, ohms check and rewrapping. Tedious work, let me tell you. I thought the dash was the hardest until I got to the headlight/battery harness. That one took the most abuse and had a lot of damage. The fusible links were really beat up. I am surprised the car did not catch fire previously after what I found.
I am still tracing, respliceing that harness. What I did find out was when the factory did joins of wires of ground, batt/alt wires, they wrapped the joint in an adhesive black duct type tape, then wrapped the harness in the non-adhesive kind. I am redoing mine the same way.
Good luck on your investigation!