I had the same problem. I put rain-X on the outside and that fixed the rain problem. The insides stay dirty due to the oils in the plastic vaporizing and precipitating on the inside of the windows.
I tried alot of things but I found a 50/50 mix of rubbing alcohol and ammonia, (make sure you do it outside on a windy day) and I used news paper as the wipe, that did a good job after about two times in a row, and then it lasted about two or three weeks before I had to do it again.
