Are you using env_weather or rain conveyors? On an env_weather, this would be caused by the sprite, if it's very dark outside then the bright sprite will appear er, bright...If possible, try reducing the fxamt on the env_weather, this will make it more transparent and might help.
To make a car shape, you'll have to do some tweaking with Vertex Manipulation and the clip tool. Experiment, and read the tutorials on them, and you should be able to figure something out.