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Hi Bam...,
The flow that was actively modified was around the rear of the truck. Look around on here, there is a long thread about this, including a link to the truck article.
According to the truck article, the airflow has to be pushed from the top, sides and bottom all at the same time to get a reduction in drag. Pushing from the top alone, causes lift and extra drag. Pushing from one sides will create a vectored thrust and more drag. Pushing air out from under the car with a fan would cause down force, and more drag.
The fans may not be designed for the speed of the airflow over a car. The blades may need to have more angle on them to be effective at highway speeds.
I am not sure how engine compartment fans could be used to reduce drag in an active fashion. Other than to allow a stagnent vent, which then would have air sucked through when the engine got too hot. But the engine fan would do this already. Pushing air out of the grill seems somewhat complicated to a simple grill block.
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