A common and cheap household item will keep the car's mirrors clean on rainy days. The toothpaste is effective for cleaning mirrors because of the fluoride found in it.
It creates friction on the surface of the mirror - meaning the water cannot condense there. The process is simple, fast and cheap - and the results are nothing less than impressive - and can even save lives. The same trick that supposedly removes scratches in a car, is expected to prevent the raindrops from accumulating on your mirrors.