No, it changes the shape of the magnetic field, causing distortion. Any metal touched by a magnet becomes magnetic itself. It is better if the magnet touches nothing that could change the overall shape of the magnetic field.
What kind of car are you putting these speakers in? www.crutchfield.com go there and find out what fits your car. There prices are high so I personally would order else where. But i'll give you an idea of what would fit.