Refer to figure.
In small piston driven aircraft like the Cessna 152 and PA28, the most common form of heating the cabin is by heating external ram air. This is normally achieved by routing the ram air through a heat exchanger, which runs around the engine exhaust system. This does not degrade engine performance in any way, or require any additional use of fuel - it simply uses heat which would otherwise be wasted through the exhaust. This indirectly heats the external air, ensuring it does not mix with the exhaust. However, a major risk from this is the possibility of a small leak in the exhaust, which may lead to carbon monoxide (CO) leaking into the cabin, severely degrading human performance or even proving fatal.

Other forms of cabin heating for light aircraft could be through a fuel-burning heater, but this is extremely inefficient and could lead to performance/range problems if not properly accounted for. It is not often seen these days. A 12 V heater would be unlikely to heat the aircraft much, and is similarly inefficient as it does not make use of the waste heat from the engine.