That's why my aux fusebox on the truck goes thru a 100amp continuous rated relay. I turn the key off, it kills everything. simples.In an effort to lessen underhood wiring and clutter, the grid heater is now built with it's control relay within. The element is stupid simple, but, the relay is still subject to failure, as is any high current switch. The failure here is from the heater being latched on until it heatsoaked enough to melt itself. With the engine running, incoming air kept it from saturating, hence the meltdown after key off.
The engineering oversight of leaving integrated, unproven, undersized, relay components "battery live" shows up in many places.
Silverado/ Sierra dash fires due to failure "latching" of the internal relay in the HVAC module.
F150s with the cruise control (subject to a national recall) due to being battery hot, and latching on when the internals fail.
I know there are many more examples, but the solution is to remove power to all this "****" when the key is turned off. Any high current device needs to NOT be battery live at key off.
If properly designed, like most of my heavy equipment, a main power relay disables ALL wiring circuits at key off, save for the 16 gauge wire to the keyswitch and a few constants to the ECMs and other devices that need power for their memory.