One problem related to turn signal has been reported for the 2007 Toyota Prius. The most recently reported issues are listed below.
The driver side headlight keeps going off, when driving the car, even with a brand new bulb in. Also, one time, both headlights went out when I was sitting at a stop sign, and turned on the turn signal. Huge safety problem! the lights come on when turned on initially then go off as I am driving on any road.