You stated new wires, but what about cables? When loading the cables have you checked for excessive heat in positive or negative?
What about painted surfaces?
And Tinstar is correct that my last problem was the starting slide on the steering column. I grabbed a known good one and replaced and it was the problem that time. I am surprised that the switch does not go bad more often as your wet or muddy shoes are always rubbing on something underdash. You say that different column same result. How about underdash wires, check? Check grounding wire strip underdash by the E-Brake foot lever.
How about the fuse box itself, have you disassembled?
How about the cable junction strip on the firewall? Rust, corrosion?
I think that what we as a collective are saying is that after a meltdown one thought at a time and thorough inspection of every wire. Voltmeter across the fusible links also.
In the dark have someone engage key and look for arching.
I believe that we have touched on about everything. Have you the CUCV wiring diagrams?
