You can, but the energy gained from the turbine will be less than the extra energy it takes to drive the vehicle now that there is a wind turbine attached.

You proposed it in the grill, but for simplicity let's put a wind turbine on the roof. To turn the blades, the wind needs to exert a force on them, and for the blades to not fly off the back of the vehicle, the vehicle needs to match that force. So now the vehicle is working harder to move forward.

It may be easier to visualize if you think of the turbine like a parachute. Once the car is up to speed, you release the turbine out of the back attached by a string. It will spin in the wind, and twist the string, but the car will slow down.

You could put this into the grill, or some part of the vehicle and fool yourself that it's not added force, but cars are designed for wind to go around them, not for wind to push on them. So no matter how you add a turbine, it's added wind resistance.