I can see a lot of people answering this with an "its impossible!"
We know it is, thats why you said "If".
Its actually quite simple.
Light is an incredible phenomena, in that regardless of where it is observed, whether it is moving toward or away, light travels at 300,000,000 meters per second in a vacuum.
Its something that can be hard to warp your mind around, but its plain and simple fact.
So here how it works.
Light travels as a wave.
It has peaks and troughs.
If a particular beam of light has a wavelength of 1 meter, and I am standing still, the light will be seen by me as light with a wavelength of one meter.
If another particular beam of light has a wavelength of one meter, and I am moving toward it at the speed of light, the light will be seen by me as having a wavelength of 0.5 meters.
And if yet another particular beam of light has a wavelength of one meter, and I am moving away from it at the speed of light, the light will be seen by me as having a wavelength of 2 meters.
Rediculous as that sounds, it is true.
So what will happen with your car, is, if you were in it, travelling at the speed of light, and turned our lights on, (and because it is white light, it emits several different wavelengths at once, appearing as white) to people observing;
all the red would become blue, all the green would become low frequency Ultraviolet, and all the blue would become mid frequency Ultra Violet.
In short, your headlights would be a deep blue.