Don’t use Bloom or any of the Lighting service Properties since you don’t want this effect to change the lighting of your entire game.
Make the Police car’s light Parts Neon so they glow then change the Brightness of the PointLight (SpotLight, SurfaceLight etc.) in them until it works properly at night. During the day it shouldn’t have as much of an effect since the Ambient Lighting Properties will tone down the contrast between day and night.
You can increase the Transparency of the white light Parts to control how much the Neon Parts glow if it’s too much.