[merseybob]

There was something about this on a tomorrows world program in the final series, now that technology was a long time ago.
It was on about house windows but the same would work on cars.

It can be made to failsafe CLEAR.
Most new cars now have electrotint rearview mirrors!

As to your comment 90% of electrical things going wrong on cars today are expensive.

So as with all the "useless additions" we have been given it would then become standard.