Use a texture which is faded so that the part of the 'light plane/mesh' close to the car is bright and the part which is far from the car is dark.
If you want shadows from any objects in front of the car, within the 'lit area' take a render from the light's perspective, make sure any shadow casting objects are colored black and then apply the resulting render to the texture, perhaps as a second layer multiply blended with the first.
If you want shadows from any objects in front of the car, within the 'lit area' take a render from the light's perspective, make sure any shadow casting objects are colored black and then apply the resulting render to the texture, perhaps as a second layer multiply blended with the first.