Here's a tough one:
How could one create a sensor which was able to detect if your taillights were on?
A way to monitor your taillight activity such as:
Let's say you have this tiny sensor which you stick to your taillight lens and, regardless of day or night or other headlights or external sources, the sensor could tell you if your taillights were working via a wireless signal.
Of course it would be even better if the sensor could tell when your brake lights or directionals came on too. I had a guy work on this for me once using a small light-sensitive "eye" normally used on laptop computers for detecting backlight, but it wasn't very reliable and easily messed up from external sources. At night and if it was positioned correctly it worked though. It would send the light activity back wirelessly to a box which mimicked the original source.
I always thought that the sensor should not operate on light-sensitive eyes but should detect the magnetic fields around the bulbs instead such as a hall sensor might do.