I have finally figured out how to do some fairly convincing rain. The method is to have a "clouds" texture applied to the background mist and make it move around with a rotating empty. I then made a rain emmiter that covered an area in front of the camera extending a little bit past the car and parented it to the camera so that it would follow the camera around. I then made a simple triangular prisim and dupliverted it with particles. It turned out pretty good.
Here is a little test animation. I left most of the layers un-rendered to save on rendering time. Video is in DivX format.
Video (146KB)

This is one of my reference pictures that I took of some big water drops comming off of my roof during a rain shower. The pictures were taken with an HP PhotoSmart C200, so that is not the best quality camera you can get, but it does the job.



I hope that you are enjoying these little "behind the scenes" tours as I go through this project. Comments are welcome!