Like in real life. Those little lights cars have so you can see plates at night. --- Post updated --- If lighting would be an issue, you could also just make a texture to overlap the plate that brightens it up a bit/makes it looks like light rays shining on the plate.
I agree that it would be a nice attention to details. In fact there is already one car from Beam which has licence plate lighting : the K-series. The quirk about this lighting on the K-series is that if you switch to high-beam, the licence plate lighting will be brighter than in low-beam.
Doesn't look right though, the light's shining on the plate but the plate itself isn't being illuminated by it. The bumper is though, lol.