"...cabin heater in the i-MiEV runs directly off the main battery, ..."
I ,m confident you know more about the MiEV traits that I do.
And I’m curious about that statement:
If I remember right the main panel ready and set a very high voltage. Something like 330 V.
I'd find it surprising if the heater coils are fed 330 volts.
If so, a damn giid reason to not be tinkering anywhere near those leads with car, let alone heater, "live" if ever trouble shooting or mantaining relative to cabin heat.