If you are required to drive with the window down on rainy days then you can't go past a Protective Plastics Weathershield.

A Protective Plastics Weathershield stops water, bugs and debris for getting on your upholstery while you are driving with the window down. This is excellent for people who do not have air-conditioning and would still like ventilation during wet weather. Protective Plastics Weathershield's are made from heavy duty acrylic so it is durable and will not wear easily.