Crummy design. A heat pump…
Crummy design. A heat pump will not "make" heat below about 40 degrees F (4.4 degrees C). Below those temperatures It will make heat by using an auxiliary source like an electric heating coil, propane or natural gas, heating oil, or a heat exchanger with an outside wood furnace (as a former home inspector, I've seen all of these). Trying to defrost / warm up a car with a heat pump will draw an enormous amount of power from the battery to heat the electric coil. Dead battery. Perhaps terminally dead.
Be glad it didn't catch on fire with anyone in it.