Most cars have wipers on the front and back windshields, and some even on the the headlights. But you'll be hard pressed to find a vehicle with wiper blades on the side mirrors which get just as wet and obscured when it's raining. So instead of just crossing your fingers and hoping you're not cutting someone off when switching lanes, try a set of these cheap $9 side mirror visors to keep them dry.
They're flexible enough to conform to the shape and contours of most mirrors, and they attach with a simple strip of 3M adhesive so you can easily apply them in a parking lot, even from the inside of your vehicle. There's no tools or custom installation required, just a basic level of hand-eye coordination. [Fancy]