Auto Parts: HVAC, guy ditch, heater boxes

Question
what is used as a source of heat for the passenger compartment?

Answer
    Well, assuming we're talking about cars, and not about air-cooled VWs, the coolant from the engine circulating through a tiny radiator mounted under the dash and called a "heater core" does the job.  There is sometimes a valve that regulates the flow of coolant through the core, or else a flap that controls the flow of air.  Then, the hot air is mixed with the A/C air to produce the desired temperature in the cabin.  If you have a VW (air cooled) or an light airplane there are heater boxes built around the exhaust manifolds which heat the air directly before routing it into the cabin.  Also, on airplanes, some of that hot air can be diverted to the carburetor's air inlet to prevent ice from forming in the carburetor.  I had this happen to an old MG being driven in humid 30-something degree weather with no air cleaners.  I also watched a guy ditch in the creek at the end of our runway at the local airport, also on a cold and humid day, for the same reason.