Variable Car Window Transparency

  Many times while driving in the sun, I wish if there was a knob in the car's dashboard, using which i could turn the windows opaque to a certain extent. Some times at night, with the tinted glasses, I wish I could turn them completely transparent.
  I dunno if such a device exists, but the laevo & dextro rotatory properties of light when passing through different mediums may be used. Another idea would be to use a chemical sandwiched between layers that could be triggered by electrical impulses.