In bright sunlight it is sometimes impossible to tell the
difference between a car indicator lamp that is on or off,
even if you're looking directly at it. Making lights brighter
than the sun would be one solution, but here's another.
A black LCD layer on top of the lamp's diffuser which is
dark when the lamp is off and transparent when it is on.
No matter how bright the ambient light it will always be
possible to tell that the indicator is blinking when looking
at it.
This still might not be enough to catch the corner of
someone's eye however, so a larger LCD layer over each
half of the boot or rear window might help, as long as it is
positioned so that the driver's rear visibility isn't affected.