There are the three ways to create a neon glow under your modifier.
1) Take mouth glow sticks and just mount them on your car, I'm not sure where to mount them though. With the mouth sticks your glow doesn't last very long and as so as its out you have to buy new glow sticks ($Waste of money$).
2) Buy some glow paint at a craft store and just paint on the bottom of the car on the undercarriage. Using glow paint is not a great way since you have to charge it up under a light for a while. Also it isn't very bright and you have to use it in total darkness.
3) The best way to give your modifier a neon under glow is to use LEDís. I'm not sure where to mount the LEDís but to see where to mount the switch and battery see LED Lights, but you need a switch (so you don't kill your battery) and a power source (such as a button battery) and holster (the thing that holds the battery in place). If you have working lights on your modifier you can just hook up the new LEDís to your existing wires. LEDís are very beneficial since they give off a lot of light, they work in broad daylight, and they don't need be charged. They can just be turned on and off as you wish.