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Under the hood might show some promise. To some degree it will act like a like a grill block.
If you want to seriously look at this its being done you are looking at the wrong end of the vehicle.
C-pillar vortices are the scourge of car aero dynamics. Not only could you recover energy from them but in doing so you could reduce drag as well. Nasa did a study on generating power from wing tip vortices and showed they could also reduce drag.
Also of note I remember reading of a study done on adding a horizontal rotating drum like structure behind a car to generate power and improve aerodynamics.
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