basically, you select the area for the "outer glow" and paint it the colour you want (neon green in this example).
then, set that layer to be a "hard light" layer.
that gives the base glow.
then, using the elipse shape, draw a few smaller and smaller elipses directly under the car, again setting the light to hard light.
the final, top elipse should be white, as this gives the massive "shine".
if that doesn't make any sense.. I'll try and do a "howto" for it