Eyeglass-darkening UV light

I wear photochromic eyeglasses (ones that darken in sunlight). They work great outdoors, but the don't work in the car because the car windows filter out the ultraviolet rays that make them darken. I could dangle them out the car window for a while to darken them, but the effect doesn't last very long and its' not very safe because it means driving with one hand and without glasses. It would be useful to have a small ultraviolet light inside the car that would automatically turn on when it is sunny outside to replace the filtered-out UV rays (except the harmful ones, of course) so that the glasses would darken just like outside.