Does the voltmeter also drop when you use other electrical items such as indicators or when headlights are turned on?

If so I would check the main earth that runs from the negative side of the battery to the body, is it clean and tight?, also I would check the main earth for the dash, from memory it connects in the panel around the inside fusebox.

Does the rear wiper move slowly or is jerky in it's movement? I would try lubing up the wiper mechanism as per Paul post.