Model T and Model A Fords used a cast iron bolt on ram air system that bolted to the exhaust manifold. The air came from the engine compartment and was less than ideal.
The Old Volkswagen being an air cooled engine, used air from the cooling. Again the air was less than ideal, especially if there was a gasket leaking
oil.
On the other hand, when a heater core leaks
antifreeze, it too is less than ideal, but that doesn't happen often.
So in short, if you picked up clean outside air and routed it through a well designed, mechanically sound, iron or steel chamber, clean warm air from the exhaust waste heat could be allowed to flow into the cockpit. I would suggest an air filter. A blower would allow heated air flow without the car needing to move.