E300 Ignition Fire

I installed a new ignition switch and lock cylinder in my 1969 E300. It started up just fine, ran just fine. But when I turned off the ignition key, the entire ignition wiring, the alternator wiring, and the voltage regulator wiring caught on fire and is currently one big molten mass of singed plastic. The auto shop I had it towed to per the auto parts store where I purchased the parts is stating that they are unable to determine exactly why the ignition fire started, so therefore they won't pay for it to be fixed.
Anybody have any ideas on how I can prove to either the auto shop or the auto parts store that it had to be the 'new' ignition switch. It's been running fine and working perfectly for the last 37 years until I changed out that ignition switch.

When you replaced it, were you ABSOLUTELY CERTAIN there were no parts./fragments of your old lock/cylinder (including circlips) in there? And, if it IS the switch making a wrongful connection/short, then it will still make that wrongful connection outside the column. remove and check! l;)

Isn't a 69's ignition switch on the dash? If so was anything hanging down behind it or laying on it. Also does that one just plug into one of those molded plastic plugs or did you have to connect the wires individually?