There are special RF absorber materials (that can be painted-like on metals) that reflect a portion of the radio signal of the outer surface and an additional amount of the inner surface against the applied to metal surface.
This paint has to be 1/4 wavelength thick. At Ka band or 33.4 GHz that would be ~2.25mm. I don't know how thick paint is typically, but I think that is thicker than typical. Forget fiber glass body parts. The metal surface is essential. If applied to fiber glass, then the engine/radiator/what ever would be the reflector & the special paint would not be effective.
But there is a range of freqs in the Ka band, 33.4 to 36 GHz. How effective this would be from end of the band to other would be interesting to study.
Of course there is the K band, 24.125 GHz. 1/4 wave thickness is 3 mm. Just a wild guess, but I'll bet that what works at Ka band does not work for K band.
Then there is X band. 10.525 GHz or 1/4 wavelength of 7 mm. Need I say more?
There are variations of these absorber materials that can be layered to add bandwidth ... but just how precisely is the thickness of paint anyway? Who knows & how can it be controlled?!?!?! Just askin'
Does laser even need to be mentioned? A little bounce off a headlight and you are tagged. A Lambo passed me today, even it as a wedge has headlights and a radiator.
Last, hypothetically suppose that a system could be made to work perfectly. All of the time. Assuming that you are the only vehicle on the road. Maybe you would be just a little bit suspicious if your car does not even register on the cop's radar as you drive past him? I'm sure this wouldn't cause you to be the most suspicious vehicle on the road. No. Not at all.