I fear it may be the harness insulation failing. Although, shutting it off seems to reset the thing. That seems to me to rule out any wires shorting but might be possible once the current draw of a device is removed. Say from a ground point as you mentioned.

And, the mechanic for the past two years has been pro bono. I'm afraid I don't have the time to replace a ETA or wiring harness.

The owner is 94 years old and I suspect it would be difficult adjusting to a different vehicle. I am hoping I can get a few more years out of this one.