I've just discovered the car comes with a heat pump (something I hear doesn't work very well even in our home homes, yet the government, and mad Ed Milliband in particular, wants us all to install one) How does this work in the context of a car.
A heat pump is just an inside out fridge.
If you look at the back of your fridge/freezer, you'll see a black radiator. That black radiator gets hot. The inside of the fridge/freezer gets cold. The whole thing runs on electricity.
So in thermodynamic terms what happens is you put in (say) 1 watt of energy to the fridge motor. That 1W of motor energy ultimatley ends up as heat somewhere. However, the motor is pumping a gas around the system, which extracts energy from the inside of the fridge (it gets colder) and dumps that energy into the back radiator (it gets hotter). So say it extracts one watt of energy from the inside of the fridge, and dumps that 1w into the radiator.
We now have a system where putting 1W of electrical energy in results in 1W of cooling, and 2W of heat (1W in the motor, 1W in the radiator). So we are 'making' more heat that the energy we are putting in. We are 'pumping heat' out of the fridge into the atmosphere. The figure of merit is called COP (Coefficient of Performance) An electric heater has a COP of 1. You put 1W of electricity in, you'll get (almost) 1W of heat out. With a heat pump you can get COPs of 3 or more. Put in 1W of electrical energy and you can get 3W (or more) of heat out.
So now mount the fridge in your wall, with the door open to the outside world, and the radiator part inside your house. Turn it on. You will be pumping heat from the outside world into your home. You won't be able to cool the outside world, so the 'inside' of the fridge won't get noticibly colder.
There are also semi-conductor based heat pump devices - called Peltiers. Here is a datasheet for one
https://tark-solutions.com/products/thermoelectric-cooler-modules/peltier-cp-series/CP2-127-06-L1-W4.5 . These don't need a pump, plumbing and gas. They are totally solid state with no moving parts, and you can get COP's in the 3-4 region. What happens is when you apply power, one side of the device gets cold, and the other side gets hot. The hot side gets hotter than the cold side gets cold, because you're 'pumping' energy (heat) from one side to the other.
Peltiers are reversible. If you swap the power the other way round, then the 'hot side' gets cold, and the 'cold side' gets hot. You can therefore put one side of peltier inside the heater (HVAC box), and use it to either heat or cool the air entering the cabin.