Undercar neon lights are not illegal. Many people do not quite understand the rules but i have found out from a mates dad who happens to be in the police force. Neon undercar kits are not illegal, so long as they follow a couple rules.
They must not flash, rotate etc.
Any direct light source (i.e. the bulbs or tubes) must not be seen, only the glow can be visable
Only white lights on the front of the car
Only red lights on the rear of the car
Only orange/amber lights on the sides of a car
I have driven past police before and none of them have stopped me; most of them just dont like them, due to the fact of being a distraction to other road users, but they are not illegal. In some states of america they are tho.