I don't know if these qualify as "economical", but it sounds like what you need is an air-to-air rotary heat exchanger, which I think are typically called enthalpy wheels or thermal wheels.

Energy Recovery Wheels | What is an Enthalpy Wheel? | DAC SALES
https://en.wikipedia.org/wiki/Thermal_wheel

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).