Before people get too upset about heat pump efficiency it would be good to think about what >100% efficiency means. The comparison is between using electric power to heat the interior using resistance, vs using some scheme with compressors, etc. to cool the environment, thereby extracting energy from it, then dumping that energy into the interior.
Think about a closed system containing the interior of the car, heated to 20C with atmospheric air and electric power as inputs, and atmospheric air as the output.
Using resistance heating, 0C atmospheric air and electric power go in, warmer air, say 5C, exits after being heated by conduction from the interior of the car. 100% of the power goes to heating the exiting atmospheric air with the interior heating as an intermediate step.
Using a heat pump, the same 0C atmospheric air and the same amount of electric power go in, however, much colder air, say -10C exits. What's going on is that the heat pump creates a local spot of say -20C which is used to extract energy from the 0C air, cooling it to -18C. That energy can then be put into the car's interior along with most of the waste heat from the heat pump. Of course the interior and some waste heat from the heat pump then heats the exiting air up to -10C, but from the outside of the box, you see 0C air going in and -10C air coming out while using the same amount of electricity.
That's how the efficiency can be said to be over 100% compared to straight resistance heating, the rest of the energy came from the cooled air exiting the box.