Stereo shops sell foam backings you can mount behind the speakers. They are bowl-shaped pieces of foam. Check with a stereo installation shop for more information. These will keep the speakers dry.

Something else that is important to know:

Modern cars have what's called a "Wet door" design. The inside of the doors get wet on purpose, and drain out through holes in the bottom of the door. There is usually some kind of a plastic barrier between the upholstery and the inner side of the door. If this "vapor barrier" is cut, removed, or broken, the inside of your car will get wet.

My VW GTI had a problem where its vapor barriers came unglued after the car was several years old. I couldn't understand where the water was coming from. The floor was SOAKED, badly enough to smell moldy in the winter. I kept trying to seal holes in the floor, not realizing the water was coming in through the doors.

Finally, I read about these door barriers and popped off the door panel to discover that they had in fact come unglued and water was streaming down the inside of the door into the car.

My point is: Whenever you do any work mounting door speakers on a car, be extremely careful not to break this seal between the door and the cabin.

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Tony Fabris