@dc42 Thanks for the tips. I will work on getting things properly grounded this weekend.
The meter indicates that the toolboard is electrically isolated from the carriage metalwork, and the carriage is isolated from the chassis of the printer, so I will tie both to a common grounding point. It's a belt-driven CoreXY, I think I need to ground my drive motors as well, as they do not have continuity to the machine chassis either.
The cartridge did not short to the case, so there's no chance it was a Vin to thermistor short. Must have been static discharge, and there have been a few zaps along the way.
Thanks again for all the help.
I have other questions about setting up multiple toolboards, but that is for another thread.